验证码的问题
我做了个生成验证码的页面code.aspx(从网上抄的)

后台cs代码如下
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.IO;
using System.Drawing.Drawing2D;

public partial class checkCode : System.Web.UI.Page
{
  protected void Page_Load(object sender, EventArgs e)
  {
  CreateCheckCodeImage(GenerateCheckCode());//调用下面两个方法;
  }

  /// <summary>
  /// 生成验证码的随机数
  /// </summary>
  /// <returns>返回五位随机数</returns>
  private string GenerateCheckCode()
  {
  int number;
  char code;
  string checkCode = String.Empty;

  Random random = new Random();

  for (int i = 0; i < 5; i++)//可以任意设定生成验证码的位数
  {
  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("CheckCode", checkCode));//写入COOKIS
  Session["CheckCode"] = checkCode; //写入Session，可以任意选一下
  return checkCode;
  }


  /// <summary>
  /// 生成验证码图片
  /// </summary>
  /// <param name="checkCode"></param>
  private void CreateCheckCodeImage(string checkCode)
  {
  if (checkCode == null || checkCode.Trim() == String.Empty)
  return;

  Bitmap image = new 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 < 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, y1, x2, y2);
  }

  Font font = new System.Drawing.Font("Arial", 12, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
  LinearGradientBrush brush = new LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
  g.DrawString(checkCode, font, brush, 2, 2);

  //画图片的前景噪音点
  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()));
  }

  //画图片的边框线
  g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

  MemoryStream ms = new MemoryStream();
  imag