asp.net ajax implements no refresh verification code

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Web; 
using System.Web.UI; 
using System.Web.UI.WebControls; 
using System.Drawing; 
namespace 学生在线考试系统 
{ 
public partial class AjaxAutoCode : System.Web.UI.Page 
{ 
//验证数字 
public string authcode = string.Empty; 
protected void Page_Load(object sender, EventArgs e) 
{ 
#region 第一种产生验证码的方法 
Random random = new Random(); 
authcode = random.Next(1111, 9999).ToString(); 
//构造图片 
Bitmap image = new Bitmap(authcode.Length * 12, 25); 
//创建画布 
Graphics g = Graphics.FromImage(image); 
try 
{ 
g.Clear(Color.White); 
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); 
//链接两点的线条 
g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2); 
} 
Font font = new Font("Arial", 12, FontStyle.Bold | FontStyle.Italic); 
System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush( 
new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkBlue, 1.2f, true); 
g.DrawString(authcode, font, brush, 2, 2); 
//画图片的前景噪点 
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())); 
} 
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); 
ms.WriteTo(this.Response.OutputStream); 
ms.Close(); 
this.Response.ContentType = "image/gif"; 
} 
finally 
{ 
image.Dispose(); 
g.Dispose(); 
} 
#endregion 
} 
} 
}

function fGetCode() 
{ 
document.getElementById("getcode").src="Default2.aspx?"+Math.random(); 
}

<label>验证码</label> 
<asp:TextBox ID="txt_checkCode" runat="server" Width="178px"></asp:TextBox> 
<img src="Default2.aspx" alt="看不清楚？" id="getcode"/> <a href="javascript:fGetCode()">更换验证码</a>