具体为什么不刷新是缓存机制不同，验证码图片的src或ImageUrl的获取是来自一个文件，由于连接地址没变所以不同内核浏览器有的会认为源没有变，解决办法就是在连接后面加上一个随机参数如可以用JS的Math.radom()

我在网上找的一个是凡的世界

http://www.cnblogs.com/Impulse/archive/2008/06/19/1226123.html

image.aspx.cs文件内容

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.Design;
using System.Drawing;
public partial class imagecode : System.Web.UI.Page
{

Random r = new Random();//声名产生随机数的变量
protected void Page_Load(object sender, 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 < 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));
System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.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);
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();
}
}

}
//验证码使用页login.aspx中的代码
<asp:ImageButton ID="yzm" runat="server" ImageUrl="imagecode.aspx" />
< asp:LinkButton ID="ChangeCode" runat="server" Text="看不清楚？换一个验证码" OnClick="ChangeCode_Click" />
< asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" /></div>
//ogin.aspx.cs中的代码
添加
//啥代码也不用写 点击“看不清楚？换一个验证码”以后就可以刷新验证码
protected void ChangeCode_Click(object sender, EventArgs e)
{

   this.ImageButton1.ImageUrl = "imagecode.aspx?id=" + r.Next();//这里是我加上的，可以解决有些浏览器不刷新问题
}
//提交按钮代码这里是通过coo