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

Graphics g = Graphics.FromImage(img);

 string s = [url=http://www.itnba.com]www.itnba.com[/url];

Font f = new Font("黑体",130);

Brush b = new SolidBrush(Color.Red);

g.DrawString(s, f, b, 5, 5);

Bitmap bit = new Bitmap(90, 40);//画布大小
    Graphics g = Graphics.FromImage(bit);//创建绘制对象，告诉它往哪张图片上绘制
    Random r = new Random();
    string s = "";
    Color color1 = Color.FromArgb(r.Next(155, 255), r.Next(155, 255), r.Next(155, 255));
    g.FillRectangle(new SolidBrush(color1), 0, 0, 90, 40);//画布颜色随机
    for (int i = 0; i < 10; i++)//随机画干扰线
    {
      Color color3 = Color.FromArgb(r.Next(170, 255), r.Next(190, 255), r.Next(170, 255));
      Pen pp = new Pen(new SolidBrush(color3), r.Next(0, 5));
      g.DrawLine(pp, r.Next(0, 90), r.Next(0, 40), r.Next(0, 90), r.Next(0, 40));
    }
    string yan = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";//设置随机的验证码，从里面抽取
    int wei = 0;
    for (int i = 0; i < 4; i++)//取四位验证码，颜色随机
    {
      string m = yan.Substring(r.Next(0, yan.Length), 1);
      s += m;
      Color color2 = Color.FromArgb(r.Next(0, 155), r.Next(0, 155), r.Next(0, 155));
      int w = r.Next(20, 25);
      Font f = new Font("黑体", w);
      SolidBrush b = new SolidBrush(color2);
      g.DrawString(m, f, b, wei, r.Next(0, 10));
      wei += w;
    }
    Session["YZM"] = s;//把验证码保存到session中
    for (int i = 0; i < 251; i++)
    {
      Color color3 = Color.FromArgb(r.Next(0, 255), r.Next(0, 255), r.Next(0, 255));
      Pen pp = new Pen(new SolidBrush(color3), r.Next(0, 5));
      int a = r.Next(0, 90);
      int b = r.Next(0, 40);
      g.DrawLine(pp, a, b, (a + 1), (b + 1));
    }
    bit.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
    Response.End();

<asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
    <asp:Image ID="Image1" runat="server" ImageUrl="Default2.aspx" />//这里图片路径不是一张图片的话会默认打开