<div>

    <asp:FileUpload ID="FileUpload1" runat="server" />

        <asp:Button ID="Button1" runat="server" Text="上传" /><br />

        <asp:Image ID="Image1" runat="server" />

</div>

//准备画布

        System.Drawing.Image img = System.Drawing.Image.FromStream(FileUpload1.FileContent);

        //创建画笔

        Graphics g = Graphics.FromImage(img);

        string s = "汉企奇点网络0928";

        //画笔的字体

        Font f = new Font("微软雅黑",);

        //画笔的样式和颜色

        Brush b = new SolidBrush(Color.Red);

        //在什么位置画

        PointF p = new PointF(,);

        //画字符串

        g.DrawString(s, f, b, p);

        string sss = "images/"+DateTime.Now.ToString("yyyyMMddhhmmssms")+FileUpload1.FileName;

        img.Save(Server.MapPath(sss));

        Image1.ImageUrl = sss;

protected void Page_Load(object sender, EventArgs e)

    {

        Bitmap img = new Bitmap(, );

        Graphics g = Graphics.FromImage(img);

        Random r = new Random();

        //创建一个颜色集合 或者也可以通过RGBA颜色直接随机生成

        List<Color> clist = new List<Color>();

        clist.Add(Color.Yellow);

        clist.Add(Color.Green);

        clist.Add(Color.Blue);

        clist.Add(Color.Pink);

        clist.Add(Color.Orange);

        clist.Add(Color.Black);

        //绘制背景色

        g.FillRectangle(new SolidBrush(clist[r.Next(, clist.Count)]), , , , );

        //循环随机绘制干扰线

        for (int i = ; i < ; i++)

        {

            Color ccc = clist[r.Next(, clist.Count)];

            Pen ppp = new Pen(new SolidBrush(ccc), r.Next(, ));

            g.DrawLine(ppp, new PointF(r.Next(, ), r.Next(, )), new PointF(r.Next(, ), r.Next(, )));

        }

        //准备一些字母数字

        string all = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

        string s = "";

        //从上面随机出四个字符拼接

        for (int i = ; i < ; i++)

        {

            int a = r.Next(all.Length);

            s += all.Substring(a, );

        }

        //存入Session

        Session["YZM"] = s;

        //绘制出验证码

        g.DrawString(s, new Font("微软雅黑", ), new SolidBrush(Color.Red), new PointF(, ));

        //通过流把绘制好的验证码输出出去

        img.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Png);

        Response.End();

    }

void Button1_Click(object sender, EventArgs e)

    {

        string t1 = TextBox1.Text;

        string t2 = Session["YZM"].ToString();

        if (t1.ToUpper() == t2.ToUpper())

        {

            Label1.Text = "验证成功！";

        }

        else

        {

            Label1.Text = "验证失败！";

        }

    }

<script type="text/javascript">

    var a = ;

    document.getElementById("Image1").onclick = function () {

        this.setAttribute("src", "yzm.aspx?id=" + a);

        a++;

    }

</script>