<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", );

        //颜色

        Brush b = new SolidBrush(Color.Red);

        //0,0——开始画水印的位置

        g.DrawString(a, f, b, , );

        //保存路径

        string path = "images/" + FileUpload1.FileName;

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

        //在image控件中展示

        Image1.ImageUrl = path;

    }

<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 = ;

    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(, );

        //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(, Clist.Count)]), , , , );

        //随机生成显示的验证码组合

        string str = "";

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

        {

            str += aaa.Substring(r.Next(, aaa.Length), );

        }

        Session["YZM"] = str;

        Font f = new Font("黑体", );

        Brush b = new SolidBrush(Color.Red);

        //生成

        g.DrawString(str, f, b, , );

        //添加干扰线

        for (int i = ; i < r.Next(, ); i++)

        {

            Brush bb = new SolidBrush(Clist[r.Next(, Clist.Count)]);

            Pen p = new Pen(bb, );

            g.DrawLine(p, r.Next(, ), r.Next(, ), r.Next(, ), r.Next(, ));

        }

        //保存完成

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

        Response.End();

    }