两者都需要引入命名空间：using System.Drawing;

一、图片水印

前台Photoshuiyin.aspx代码：

<div>

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

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

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

</div>

后台Photoshuiyin.aspx.cs代码：

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;

}

效果展示：

二、图片验证码

前台Photoyanzhengma.aspx代码：

<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>

链接页面“YZM.aspx”——无需前台代码，后台代码是：

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();

}

效果展示：