using System.Drawing.Imaging;
using System.IO;

   //生成4位的验证码
   string tmp = RndNum(4);
    HttpCookie a = new HttpCookie("ImageV",tmp);   
    Response.Cookies.Add(a);
   this.ValidateCode(tmp);

private void ValidateCode(string VNum)
  {
    Bitmap Img=null;
    Graphics g=null;
    MemoryStream ms=null;

   int gheight=VNum.Length*12;
    Img=new Bitmap(gheight,25);
    g=Graphics.FromImage(Img);
   //背景颜色
    g.Clear(Color.White);
   //文字字体
    Font f=new Font("Arial Black",10);
   //文字颜色
    SolidBrush s=new SolidBrush(Color.Black);
    g.DrawString(VNum,f,s,3,3);
    ms=new MemoryStream();
    Img.Save(ms,ImageFormat.Jpeg);
    Response.ClearContent();
    Response.ContentType="image/Jpeg";
    Response.BinaryWrite(ms.ToArray());
   
    g.Dispose();
    Img.Dispose();
    Response.End();
   }

  private string RndNum(int VcodeNum)
  {
   string Vchar="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,v,w,x,y,z";
   string[] VcArray=Vchar.Split(new Char [] {','});
   string VNum="";
   int temp=-1;

    Random rand=new Random();
   
   for(int i=1;i<VcodeNum+1;i++)
   {
    if(temp!=-1)
    {
      rand=new Random(i*temp*unchecked((int)DateTime.Now.Ticks));
     }
   
    int t=rand.Next(35);
    if(temp!=-1&&temp==t)
    {
     return RndNum(VcodeNum);
     }
     temp=t;
     VNum+=VcArray[t];
    }
   return VNum;
   }

ImageButton1.ImageUrl = "image.aspx";