private void CreateImage(string checkCode)
        {
            int iwidth = (int)(checkCode.Length * 15);
            System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 25);
            Graphics g = Graphics.FromImage(image);
            g.Clear(Color.White);
//定义颜色
            Color[] c = {Color.Black,Color.Red,Color.DarkBlue,Color.Green,Color.Orange,Color.Brown,Color.DarkCyan,Color.Purple};
//定义字体            string[] font = {"Verdana","Microsoft Sans Serif","Comic Sans MS","Arial","宋体"};
            Random rand = new Random();
//随机输出噪点
            for(int i=0;i<50;i++)
            {
                int x = rand.Next(image.Width);
                int y = rand.Next(image.Height);
                g.DrawRectangle(new Pen(Color.LightGray, 0),x,y,1,1);
            }

//输出不同字体和颜色的验证码字符
            for(int i=0;i<checkCode.Length;i++)
            {
                int cindex = rand.Next(7);
                int findex = rand.Next(5);
               
                Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
                Brush b = new System.Drawing.SolidBrush(c[cindex]);
                int ii=4;
                if((i+1)%2==0)
                {
                    ii=2;
                }
                g.DrawString(checkCode.Substring(i,1), f, b, 3+(i*12), ii);
            }
//画一个边框
            g.DrawRectangle(new Pen(Color.Black,0),0,0,image.Width-1,image.Height-1);

//输出到浏览器
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
            Response.ClearContent();
            Response.ContentType = "image/Jpeg";
            Response.BinaryWrite(ms.ToArray());
            g.Dispose();
            image.Dispose();
        }

（出处：CodeFans转载）