放验证码的页面内添加如下js脚本，这个是刷新验证码的方法：

<script language="javascript" type="text/javascript">  
function fresh()  
{  
var randomnum=Math.random();  
var getimagecode=document.getElementById( "codeimage");  
getimagecode.src= "ValidateImage.aspx? "+randomnum;  
}  
</script>  


//该代码片段来自于: http://www.sharejs.com/codes/csharp/7816

页面内要放验证码的地方（层内，单元格内等）放置如下代码，这个是验证码的图图，输入验证码的文本框和调刷新方法的链接。

<div><img id="codeimage" src="ValidateImage.aspx?,Math.random()" />  
<asp:TextBox ID="txtValidateCode" runat="server" Width="55px" MaxLength="4"></asp:TextBox>  
<a href="javascript:fresh()" style="font-size: 12px; color: Green">看不清</a></div>  


//该代码片段来自于: http://www.sharejs.com/codes/csharp/7816

Validateimage.aspx页面是生成图片

private void Page_Load(object sender, System.EventArgs e)  
   {  
       this.CreateCheckCodeImage(RndNum());  
   }  
   private string RndNum()  
   {  
       int number;  
       char code;  
       string checkCode = String.Empty;  
  
       System.Random random = new Random();  
  
       for (int i = 0; i < 4; i++)  
       {  
           number = random.Next();  
           if (number % 2 == 0)  
               code = (char)('0' + (char)(number % 10));  
           else  
               code = (char)('A' + (char)(number % 26));  
           checkCode += code.ToString();  
       }  
       Response.Cookies.Add(new HttpCookie("yzmcode", checkCode));  
       return checkCode;  
   }  
   private void CreateCheckCodeImage(string checkCode)  
   {  
       if (checkCode == null || checkCode.Trim() == String.Empty)  
           return;  
       System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length * 12.5)), 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 < 20; 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.Aqua), 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.BlueViolet, Color.BlueViolet, 1.2f, true);  
           g.DrawString(checkCode, font, brush, 2, 2);  
           //画图片的前景噪音点   
           for (int i = 0; i < 50; 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();  
           image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);  
           Response.ClearContent();  
           Response.ContentType = "image/Gif";  
           Response.BinaryWrite(ms.ToArray());  
       }  
       finally  
       {  
           g.Dispose();  
           image.Dispose();  
       }  
   }