js源码：

/// <reference path="jquery-1.8.3.js" />
/// <reference path="ajaxForm/jquery.form.js" />

/*!
 * jQuery upload
 * 用于上传单个文件，上传成功后,返回文件路径。
 * 本插件依赖jQuery，jquery.form 请在使用时引入依赖的文件
 *
 * Date: 2014/4/23
 */
/*
使用:
html:
<div style="width: 100%; float: left;">
    <input type="hidden" id="hfThumbnail" value="/uploads/2014/04/23/635338686611432716.jpg" />
    <div class="imgdiv"></div>
    <span class="img_span">
        <input type="file" name="file" />
    </span>
     
    <input type="button" value="上传" class="upload" />
</div>
<div style="width: 100%; float: left;">
    <input type="hidden" id="hfThumbnail" value="/uploads/2014/04/23/635338686611432716.jpg" />
    <div class="imgdiv"></div>
    <span class="img_span">
        <input type="file" name="file" />
    </span>
     
    <input type="button" value="上传" class="upload" />
</div>

js:

$(document).ready(function () {
        
    //$(".upload").upload({
    //    uploadData: { id: "12233" },
    //    successFn: function (response, statusText, xhr, $this) {
    //        alert(response.Data.RelativePath)
    //    },
    //    deleteData: { id: function () { return "asdfasdf" } }
    //});
  
    $(".upload").upload({
        uploadData: { id: "12233" },
        successFn: "success",           //可以是字符串
        deleteData: { id: function () { return "asdfasdf" } }
    });
});

//上传成功后执行该函数
function success(response, statusText, xhr, $this) {
    //比如插入编辑器
    alert(response.Data.RelativePath + $this.attr("value"))
}
*/

(function ($) {
    $.extend($.fn, {
        upload: function (settings) {

            var options = $.extend({
                fileType: "gif|jpg|jpeg|png|bmp",                       //允许的文件格式
                uploadUrl: "/ajax/handler.ashx?action=uploadfile",      //上传URL地址
                deleteUrl: "/ajax/handler.ashx?action=deletefile",      //删除URL地址
                width: "",                                              //图片显示的宽度
                height: 100,                                            //图片显示的高度
                imgSelector: ".imgdiv",                                  //图片选择器
                uploadData: {},                                         //上传时需要附加的参数
                deleteData: {},                                         //删除时需要附加的参数
                deleteFn: function ($parent, showMessage) {             //删除图片的方法(默认方法使用POST提交)
                    methods.deleteImage($parent, showMessage);
                },
                beforeSubmitFn: "beforeUpload",                         //上传前执行的方法 原型 beforeSubmit(arr, $form, options);
                successFn: "uploadSuccess",                             //上传成功后执行的方法 uploadSuccess(response, statusText, xhr, $this)
                errorFn: "uploadError"                                  //上传失败后执行的方法
            }, settings);

            //上传准备函数
            var methods = {
                //验证文件格式
                checkFile: function (filename) {
                    var pos = filename.lastIndexOf(".");
                    var str = filename.substring(pos, filename.length);
                    var str1 = str.toLowerCase();
                    if (typeof options.fileType !== ‘string‘) { options.fileType = "gif|jpg|jpeg|png|bmp"; }
                    var re = new RegExp("\.(" + options.fileType + ")$");
                    return re.test(str1);
                },
                //创建表单
                createForm: function () {
                    var $form = document.createElement("form");
                    $form.action = options.uploadUrl;
                    $form.method = "post";
                    $form.enctype = "multipart/form-data";
                    $form.style.display = "none";
                    //将表单加当document上，
                    document.body.appendChild($form);  //创建表单后一定要加上这句否则得到的form不能上传。document后要加上body,否则火狐下不行。
                    return $($form);
                },
                //创建图片
                createImage: function () {
                    //不能用 new Image() 来创建图片，否则ie下不能改变img 的宽高
                    var img = $(document.createElement("img"));
                    img.attr({ "title": "双击图片可删除图片！" });
                    if (options.width !== "") {
                        img.attr({ "width": options.width });
                    }
                    if (options.height !== "") {
                        img.attr({ "height": options.height });
                    }
                    return img;
                },
                showImage: function (filePath, $parent) {
                    var $img = methods.createImage();
                    $parent.find(options.imgSelector).find("img").remove();
                    //要先append再给img赋值，否则在ie下不能缩小宽度。
                    $img.appendTo($parent.find(options.imgSelector));
                    $img.attr("src", filePath);
                    this.bindDelete($parent);
                },
                bindDelete: function ($parent) {
                    $parent.find(options.imgSelector).find("img").bind("dblclick", function () {
                        options.deleteFn($parent, true);
                    });
                },
                deleteImage: function ($parent, showMessage) {
                    var $fileInput = $parent.find("input:hidden");
                    if ($fileInput.val() !== "") {

                        var data = $.extend(options.deleteData, { filePath: $fileInput.val(), t: Math.random() });

                        $.post(options.deleteUrl, data, function (response) {

                            if (showMessage) { alert(response.MessageContent) }

                            if (response.MessageType == 1) {
                                $fileInput.val("");
                                $parent.find(options.imgSelector).find("img").remove();
                            }
                        }, "JSON");
                    }
                },
                onl oad: function ($parent) {
                    var hiddenInput = $parent.find("input:hidden");
                    if (typeof hiddenInput !== "undefined" && hiddenInput.val() !== "") {
                        var img = methods.createImage();
                        if ($parent.find(options.imgSelector).find("img").length > 0) { $parent.find(options.imgSelector).find("img").remove(); }
                        //要先append再给img赋值，否则在ie下不能缩小宽度。 
                        img.appendTo($parent.find(options.imgSelector));
                        img.attr("src", hiddenInput.val());
                        methods.bindDelete($parent);
                    }
                }
            };
            //上传主函数
            this.each(function () {
                var $this = $(this);
                methods.onload($this.parent());
                $this.bind("click", function () {
                    var $fileInput = $(this).parent().find("input:file");
                    var fileBox = $fileInput.parent();

                    if ($fileInput.val() === "") {
                        alert("请选择要上传的图片！");
                        return false;
                    }
                    //验证图片
                    if (!methods.checkFile($fileInput.val())) {
                        alert("文件格式不正确，只能上传格式为：" + options.fileType + "的文件。");
                        return false;
                    }
                    //若隐藏域中有图片，先删除图片
                    if ($fileInput.val() !== "") {
                        options.deleteFn($this.parent(), false);
                        //methods.deleteImage($this.parent(), false);
                    }

                    //创建表单
                    var $form = methods.createForm();

                    //把上传控件附加到表单
                    $fileInput.appendTo($form);
                    fileBox.html("<img src=\"/admin/styles/images/loading.gif\" />   正在上传...  ");
                    $this.attr("disabled", true);

                    //构建ajaxSubmit参数
                    var data = {};
                    data.data = options.uploadData;
                    data.type = "POST";
                    data.dataType = "JSON";
                    //上传前
                    data.beforeSubmit = function (arr, $form, options) {

                        var beforeSubmitFn;
                        try { beforeSubmitFn = eval(options.beforeSubmitFn) } catch (err) { };
                        if (beforeSubmitFn) {
                            var $result = beforeSubmitFn(arr, $form, options);
                            if (typeof ($result) == "boolean")
                                return $result;
                        }
                    };
                    //上传失败
                    data.error = function (response, statusText, xhr, $form) {
                        var errorFn;
                        try { errorFn = eval(options.errorFn) } catch (err) { };
                        if (errorFn) {
                            errorFn(response.responseText, statusText, xhr, $this);
                        }
                    };
                    //上传成功
                    data.success = function (response, statusText, xhr, $form) {
                        //response = eval("(" + response + ")");
                        if (response.MessageType == 1) {
                            methods.showImage(response.Data.RelativePath, $this.parent());
                            $this.parent().find("input:hidden").val(response.Data.RelativePath);

                            var successFn;
                            try { successFn = eval(options.successFn) } catch (err) { };
                            if (successFn) {
                                $.ajaxSetup({ cache: false });//这个不能少，否则ie下会提示下载
                                successFn(response, statusText, xhr, $this);
                            }


                        } else {
                            alert(response.MessageContent);
                        }
                        $this.attr("disabled", false);
                        fileBox.html("<input type=\"file\" name=\"file\" />");
                        $form.remove();
                    };

                    try {
                        //开始ajax提交表单
                        $form.ajaxSubmit(data);
                    } catch (e) {
                        alert(e.message);
                    }
                });
            });
        }
    });
})(jQuery)

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script src="Scripts/jquery/jquery-1.8.3.js"></script>
<script src="Scripts/jquery/ajaxForm/jquery.form.js"></script>
<script src="Scripts/jquery/jquery.upload.js"></script>
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div style="width: 100%; float: left;">
    <input type="hidden" id="hfThumbnail" value="/uploads/2014/04/23/635338686611432716.jpg" />
    <div class="imgdiv"></div>
    <span class="img_span">
        <input type="file" name="file" />
    </span>
     
    <input type="button" value="上传" class="upload" />
</div>
<div style="width: 100%; float: left;">
    <input type="hidden" id="hfThumbnail" value="/uploads/2014/04/23/635338686611432716.jpg" />
    <div class="imgdiv"></div>
    <span class="img_span">
        <input type="file" name="file" />
    </span>
     
    <input type="button" value="上传" class="upload" />
</div>
</form>
<script type="text/javascript">
    $(document).ready(function () {
        //$(".upload").upload({
        //    uploadData: { id: "12233" },
        //    successFn: function (response, statusText, xhr, $this) {
        //        alert(response.Data.RelativePath)
        //    },
        //    deleteData: { id: function () { return "asdfasdf" } }
        //});
        $(".upload").upload({
            uploadData: { id: "12233" },
            successFn: "success",
            deleteData: { id: function () { return "asdfasdf" } }
        });
    });

    //上传成功后执行该函数
    function success(response, statusText, xhr, $this) {
        //比如插入编辑器
        alert(response.Data.RelativePath + $this.attr("value"))
    }
</script>
</body>
</html>

public void ProcessRequest(HttpContext context)
{
    context.Response.ContentType = "application/json";
    var action = context.Request.QueryString.Get<string>("action").ToLower();
    var response = new JsonResult(StatusMessageType.Error, "没有权限或无效请求。").ToJson();
    switch (action)
    {

        case "uploadfile":
            if (context.Request.Files.Count > 0)
                response = UploadFile(context.Request);
            break;
        case "deletefile":
            response = DeleteFile(context.Request.Form);
            break;
        default:
            break;
    }
    context.Response.Write(response);
}
/// <summary>
/// 
/// </summary>
/// <param name="file"></param>
/// <returns></returns>
private string UploadFile(HttpRequest request)
{
    if (request.Files.Count == 0)
        return new JsonResult(StatusMessageType.Error, "没有可处理的数据。").ToJson();
    var id = request.Form.Get<string>("id", "");

    var file = request.Files[0];
    if (file == null || file.ContentLength <= 0 || string.IsNullOrEmpty(file.FileName))
        return new JsonResult(StatusMessageType.Error, "没有可处理的数据。").ToJson();

    //IStoreFile storeFile = null;

    string[] datePaths = new string[] { "~/uploads/" };
    datePaths = datePaths.Union(DateTime.Now.ToString("yyyy-MM-dd").Split(‘-‘)).ToArray();
    var relativePath = storeProvider.JoinDirectory(datePaths);

    var dirPath = HttpContext.Current.Server.MapPath(relativePath);

    if (!System.IO.Directory.Exists(dirPath))
        System.IO.Directory.CreateDirectory(dirPath);

    var fileName = GenerateFileName(Path.GetExtension(file.FileName));

    var filePath = Path.Combine(dirPath, fileName);
    file.SaveAs(filePath);
    return new JsonResult(StatusMessageType.Success, "上传成功。", new
    {
        RelativePath = WebUtility.ResolveUrl(Path.Combine(relativePath, fileName)),
        Size = file.ContentLength,
        Id = id,
    }).ToJson();
}

public string DeleteFile(NameValueCollection form)
{
    var filePath = form.Get<string>("filePath", "").Trim();
    if (string.IsNullOrEmpty(filePath))
        return new JsonResult(StatusMessageType.Error, "无效提交。").ToJson();

    try
    {
        if (System.IO.File.Exists(HttpContext.Current.Server.MapPath(filePath)))
        {
            System.IO.File.Delete(HttpContext.Current.Server.MapPath(filePath));
            return new JsonResult(StatusMessageType.Success, "文件删除成功。").ToJson();
        }
        else
        {
            return new JsonResult(StatusMessageType.Success, "找不到该文件。").ToJson();
        }
    }
    catch (Exception)
    {
        return new JsonResult(StatusMessageType.Hint, "发生错误。").ToJson();
    }
}

/// <summary>
/// 生成随机文件名
/// </summary>
/// <returns></returns>
private string GenerateFileName(string extension)
{
    return DateTime.Now.Ticks.ToString() + extension;
}

[Serializable]
public class JsonResult
{
    private StatusMessageType _messageType;
    private string _messageContent;


    /// <summary>
    /// 
    /// </summary>
    /// <param name="messageType"></param>
    /// <param name="messageContent"></param>
    /// <param name="data"></param>
    public JsonResult(StatusMessageType messageType, string messageContent, object data = http://www.mamicode.com/null)>
StatusMessageType枚举类：
/// <summary>
/// 提示消息类别
/// </summary>
public enum StatusMessageType
{
    /// <summary>
    /// 权限不足
    /// </summary>
    NoAccess = -2,
    /// <summary>
    /// 错误
    /// </summary>
    Error = -1,
    /// <summary>
    /// 成功
    /// </summary>
    Success = 1,

    /// <summary>
    /// 提示信息
    /// </summary>
    Hint = 0
}

其他的扩展方法就不一一给出来了。





ajax图片上传，图片异步上传，更新