<form id="form1" runat="server">
    <div>
            用户名：<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            <br />
            密码：<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
            <br />
            验证码：<asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
            <asp:Image ID="Image1" runat="server" ImageUrl="YZM.aspx" />
            <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
            <br />
            <asp:Button ID="Button1" runat="server" Text="Button" />
    </div>
    </form>
</body>
</html>
<script type="text/javascript">
    var aaa = 1;
    document.getElementById("Image1").onclick = function () {
        this.setAttribute("src", "YZM.aspx?id=" + aaa);
        aaa++;
    };
</script>

protected void Page_Load(object sender, EventArgs e)
    {
        Random r = new Random();
        string aaa = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        //生成画布
        Bitmap img = new Bitmap(80, 30);
        //画布背景色泛性组合
        List<Color> Clist = new List<Color>();
        Clist.Add(Color.Yellow);
        Clist.Add(Color.Green);
        Clist.Add(Color.Blue);
        Clist.Add(Color.Aqua);
        Clist.Add(Color.Orange);
        Clist.Add(Color.Pink);

        Graphics g = Graphics.FromImage(img);

        g.FillRectangle(new SolidBrush(Clist[r.Next(0, Clist.Count)]), 0, 0, 80, 30);
        //随机生成显示的验证码组合
        string str = "";
        for (int i = 0; i < 4; i++)
        {
            str += aaa.Substring(r.Next(0, aaa.Length), 1);
        }

        Session["YZM"] = str;
        Font f = new Font("黑体", 20);
        Brush b = new SolidBrush(Color.Red);
        //生成
        g.DrawString(str, f, b, 10, 0);
        //添加干扰线
        for (int i = 0; i < r.Next(6, 20); i++)
        {
            Brush bb = new SolidBrush(Clist[r.Next(0, Clist.Count)]);
            Pen p = new Pen(bb, 1);
            g.DrawLine(p, r.Next(0, 80), r.Next(0, 30), r.Next(0, 80), r.Next(0, 30));
        }
        //保存完成
        img.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
        Response.End();
    }