<div>
    <asp:FileUpload ID="FileUpload1" runat="server" />
        <asp:Button ID="Button1" runat="server" Text="上传" /><br />
        <asp:Image ID="Image1" runat="server" />
    </div>

protected void Page_Load(object sender, EventArgs e)
    {
        Button1.Click += Button1_Click;
    }

    void Button1_Click(object sender, EventArgs e)
    {
        //1、制作画布
        System.Drawing.Image img = System.Drawing.Image.FromStream(FileUpload1.FileContent);

        Graphics g = Graphics.FromImage(img);
        //水印样式：画什么东西
        string a = "http://www.itnba.com";
        //字体、大小
        Font f = new Font("黑体", 30);
        //颜色
        Brush b = new SolidBrush(Color.Red);
        //0,0——开始画水印的位置
        g.DrawString(a, f, b, 0, 0);
        //保存路径
        string path = "images/" + FileUpload1.FileName;
        img.Save(Server.MapPath(path));
        //在image控件中展示
        Image1.ImageUrl = path;

    }