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 = 14;//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 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
{
int n = random.Next(10);//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
}

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(/upload/tech/20091011/20091011145015_6512bd43d9caa6e02c990b0a82652dca.gif);//保存图像，等待输出
}

<asp:Image ID="Image1" runat="server" Height="21px" Width="61px" ImageUrl="~/Default2.aspx" ImageAlign="Middle" />