关于aspx后台生成验证码图片的问题，请高手指点。

在网上找的代码，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.Drawing.Imaging;
using System.IO;namespace CheckCode
{
    public partial class Default: System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            string randomcode = this.CreateRandomCode(4);
            //Response.Cookies.Add(new HttpCookie("CheckCode", randomcode));
            Session["ValidateCode"] = randomcode; // Session、Cookie、ViewState都可保存，根据实际情况定  
            this.CreateImage(randomcode);
        }
        ///  <summary>  
        ///  生成随机码  
        ///  </summary>  
        ///  <param  name="length">随机码个数</param>  
        ///  <returns>随机码</returns>  
        private string CreateRandomCode(int length)
        {
            int rand;
            char code;
            string randomcode = String.Empty;            //生成一定长度的验证码  
            System.Random random = new Random();
            for (int i = 0; i < length; i++)
            {
                rand = random.Next();                if (rand % 3 == 0)
                {
                    code = (char)('A' + (char)(rand % 26));
                }
                else
                {
                    code = (char)('0' + (char)(rand % 10));
                }                randomcode += code.ToString();
            }
            return randomcode;
        }        ///  <summary>  
        ///  创建随机码图片  
        ///  </summary>  
        ///  <param  name="randomcode">随机码</param>  
        private void CreateImage(string randomcode)
        {            int randAngle = 45;     //随机转动角度  
            int mapwidth = (int)(randomcode.Length * 16);
            Bitmap map = new Bitmap(mapwidth, 28);  //创建图片背景，设置其长宽  
            Graphics graph = Graphics.FromImage(map);
            graph.Clear(Color.AliceBlue);
            graph.DrawRectangle(new Pen(Color.Black, 0), 0, 0, map.Width - 1, map.Height - 1);//画一个边框              Random rand = new Random();            // 生成背景噪点  
            Pen blackPen = new Pen(Color.LightGray, 0);
            for (int i = 0; i < 50; i++)
            {
                int x = rand.Next(0, map.Width);
                int y = rand.Next(0, map.Height);
                graph.DrawRectangle(blackPen, x, y, 1, 1);
            }            //验证码旋转，防止机器识别  
            char[] chars = randomcode.ToCharArray();//拆散字符串成单字符数组              //文字距中  
            StringFormat format = new StringFormat(StringFormatFlags.NoClip);
            format.Alignment = StringAlignment.Center;
            format.LineAlignment = StringAlignment.Center;            // 定义随机颜色列表  
            Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
            // 定义随机字体字体  
            string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };            for (int i = 0; i < chars.Length; i++)
            {
                int cindex = rand.Next(7);
                int findex = rand.Next(5);                Font f = new System.Drawing.Font(font[findex], 16, System.Drawing.FontStyle.Bold); // 字体样式(参数2为字体大小)  
                Brush b = new System.Drawing.SolidBrush(c[cindex]);                Point dot = new Point(11, 11);  // 括号内数值越大，字符间距越大  
                float angle = rand.Next(0, randAngle);  // 转动的度数，如果将0改为-randAngle，那么旋转角度为-45度～45度                  graph.TranslateTransform(dot.X, dot.Y);
                graph.RotateTransform(angle);
                graph.DrawString(chars[i].ToString(), f, b, 2, 6, format); // 第4、5个参数控制左、上间距  
                graph.RotateTransform(-angle);
                graph.TranslateTransform(2, -dot.Y);
            }            //生成图片  
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            map.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
            //Response.ClearContent();
            //Response.ContentType = "image/gif";
            //Response.BinaryWrite(ms.ToArray());
            graph.Dispose();
            map.Dispose();
            
        }
    }
}前台代码如下：<img src="Default.aspx" style="vertical-align:bottom; margin-bottom:1px; cursor: pointer;" alt="点击刷新" onclick="刷新当前页面的javascript函数" />问题是，后台代码里边直接就有response.binarywrite()，load的时候前台页面就被写好了，前台代码写什么也没用啊，怎么调用，把生成的验证码图片放到任意位置？请指教