form id="form1" runat="server">
  div>
    asp:TextBox ID="TextBox1" runat="server">/asp:TextBox>
    asp:Image ID="Image1" imageUrl=“/default.aspx” runat="server" />
  /div>
/form>

using System;
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 _Default : System.Web.UI.Page 
{
  protected void Page_Load(object sender, EventArgs e)
  {
    if (!IsPostBack)
    {
      string validateNum = CreateRandomNum(4);
      CreateImage(validateNum);
      Session["ValidateNum"] = validateNum;
    }
  }
  //生产随机数
  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,O,P,Q,R,S,T,U,W,X,Y,Z,a,b,c,d,e,f,g,h,i,j,k,m,n,o,p,q,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;
  }
  //生产图片
  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+12,22);
    Graphics g = Graphics.FromImage(image);
    try 
    { 
      //生成随机生成器
      Random random = new Random();
      //清空图片背景
      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,x2,y1,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;i100;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();
      //将图像保存到指定流
      image.Save(ms,System.Drawing.Imaging.ImageFormat.Gif);
      Response.ClearContent();
      Response.ContentType="image/Gif";
      Response.BinaryWrite(ms.ToArray());
    }
    finally
    {
      g.Dispose();
      image.Dispose();
    }
  }
}