asp:Image ID="Image1" runat="server"ImageUrl="~/ValidateNum.aspx" />
asp:LinkButton ID="LinkButton1" runat="server" style="font-size: small; ">看不清，换一张/asp:LinkButton>

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Drawing;

public partial class ValidateNum : System.Web.UI.Page
{
 protected void Page_Load(object sender, EventArgs e)
 {
  if (!IsPostBack)
  {
   string validateNum = CreateRandomNum(4);//成生4位随机字符串
   CreateImage(validateNum);//将生成的随机字符串绘成图片
   Session["ValidateNum"] = validateNum;//保存验证码
  }
 }
 //生成随机字符串
 private string CreateRandomNum(int NumCount)
 {
  string allChar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,W,X,Y,Z";
  string[] allCharArray = allChar.Split(',');//拆分成数组
  string randomNum = "";
  int temp = -1;//记录上次随机数的数值，尽量避免产生几个相同的随机数

  Random rand = new Random();
  for (int i = 0; i  NumCount; i++)
  {
   if (temp != -1)
   {
    rand = new Random(i * temp * ((int)DateTime.Now.Ticks));
   }
   int t = rand.Next(35);
   if (temp == t)
   {
    return CreateRandomNum(NumCount);
   }
   temp = t;
   randomNum += allCharArray[t];
  }
  return randomNum;
 }
 //生成图片
 private void CreateImage(string validateNum)
 {
  if (validateNum == null || validateNum.Trim() == String.Empty)
   return;
  //生成Bitmap图像
  System.Drawing.Bitmap image = new System.Drawing.Bitmap(validateNum.Length * 12 + 10, 22);
  Graphics g = Graphics.FromImage(image);

  try
  {
   //生成随机生成器 
   Random random = new Random();

   //清空图片背景色 
   g.Clear(Color.White);

   //画图片的背景噪音线 
   for (int i = 0; i  25; i++)
   {
    int x1 = random.Next(image.Width);
    int x2 = random.Next(image.Width);
    int y1 = random.Next(image.Height);
    int y2 = random.Next(image.Height);

    g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);
   }

   Font font = new System.Drawing.Font("Arial", 12, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
   System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
   g.DrawString(validateNum, font, brush, 2, 2);

   //画图片的前景噪音点 
   for (int i = 0; i  100; i++)
   {
    int x = random.Next(image.Width);
    int y = random.Next(image.Height);

    image.SetPixel(x, y, Color.FromArgb(random.Next()));
   }

   //画图片的边框线 
   g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

   System.IO.MemoryStream ms = new System.IO.MemoryStream();
   //将图像保存到指定的流
   image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
   Response.ClearContent();
   Response.ContentType = "image/Gif";
   Response.BinaryWrite(ms.ToArray());
  }
  finally
  {
   g.Dispose();
   image.Dispose();
  }
 }
}