方法一：点击图片实现更换验证码

<img src="验证码界面路径" alt="看不清楚，换一张" id="img" οnclick="img.src=验证码界面路径?'+new Date().getTime()";" />

方法二：点超链接更换验证码、下面这个例子两个方法都用到了、点击图片也可以

<script language="javascript">  

function change()  

{  

var img =document.getElementById("codeimg");  

img.src=img.src+"?";  

}  

</script>  

<a href="javascript:change();">看不清,换一张 </a>  

<img alt="" src="" style="cursor:pointer;" id="codeimg" οnclick="this.src='validate.aspx?time=' + Math.random();" /> <script language="javascript" type="text/javascript">document.getElementById('codeimg').src = 'validate.aspx?time=' + Math.random(); </script>  

三、

protected void Page_Load(object sender, EventArgs e)

{

string validateCode = CreateValidateCode();//生成验证码

Bitmap bitmap = new Bitmap(imgWidth, imgHeight);//生成Bitmap图像

DisturbBitmap(bitmap); //图像背景

DrewValidateCode(bitmap, validateCode);//绘制验证码图像

bitmap.Save(Response.OutputStream, ImageFormat.Gif);//保存图像，等待输出

}

private int codeLen = 4;//验证码长度

private int fineness = 85;//图片清晰度

private int imgWidth = 48;//图片宽度

private int imgHeight = 24;//图片高度

private string fontFamily = "Times New Roman";//字体名称

private int fontSize = 10;//字体大小

private int fontStyle = 0;//字体样式

private int posX = 0;//绘制起始坐标X

private int posY = 0;//绘制坐标Y

private string CreateValidateCode() //生成验证码

{

string[] source = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P,", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };

string validateCode = "";

Random random = new Random();// 随机数对象

for (int i = 0; i < codeLen; i++)//循环生成每位数值

{

string n =source[random.Next(0,source.Length)] ;//数字

validateCode += n.ToString();

}

Session["vcode"] = validateCode;//保存验证码

return validateCode;// 返回验证码

}

private void DisturbBitmap(Bitmap bitmap)//图像背景

{

Random random = new Random();//通过随机数生成

for (int i = 0; i < bitmap.Width; i++)//通过循环嵌套，逐个像素点生成

{

for (int j = 0; j < bitmap.Height; j++)

{

if (random.Next(90) <= this.fineness)

bitmap.SetPixel(i, j, Color.LightGray);

}

}

}

private void DrewValidateCode(Bitmap bitmap, string validateCode)//绘制验证码图像

{

Graphics g = Graphics.FromImage(bitmap);//获取绘制器对象

Font font = new Font(fontFamily, fontSize, FontStyle.Bold);//设置绘制字体

g.DrawString(validateCode, font, Brushes.Black, posX, posY);//绘制验证码图像

}