验证码--点击验证码就换一个验证码---最简单的

html页面

  <form id="form1" runat="server">   
  <img src="Handler2.ashx" onclick="this.src='Handler2.ashx?aaa='+new Date()" />
    </form>

ashx页面，一般处理程序页面：

 public class Handler2 : IHttpHandler
    {

        public void ProcessRequest(HttpContext context)
        {
            //context.Response.ContentType = "text/html";//返回一个text的东西，也就是string
            context.Response.ContentType = "image/JPEG";
            using (System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(100, 50))//(100,50)是大小
            {
                using(System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap) )
                {
                    Random rand = new Random();
                    int code = rand.Next(1111, 2222);
                    string mycode = code.ToString();
                   // HttpContext.Current.Session["code_yes"] = mycode;
                    g.DrawString(mycode,new System.Drawing.Font("宋体",50),System.Drawing.Brushes.Green,new System.Drawing.PointF(0,0));
                    bitmap.Save(context.Response.OutputStream,System.Drawing.Imaging.ImageFormat.Jpeg);
                }
            }
           
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }