protected void Page_Load(object sender, EventArgs e)
    {
        System.Drawing.Bitmap img = new System.Drawing.Bitmap(300, 100);
        System.Drawing.Graphics graphics = System.Drawing.Graphics.FromImage(img);
       
        System.Drawing.Font font = new System.Drawing.Font("宋体", 16); //字体与大小
        System.Drawing.Brush brush = new System.Drawing.SolidBrush(System.Drawing.Color.Red);
        graphics.DrawString("www.cftea.com", font, brush, 50, 50); //写字，最后两个参数表示位置
       
        //将图片保存到内存流中
        System.IO.MemoryStream stream = new System.IO.MemoryStream();
        img.Save(stream, System.Drawing.Imaging.ImageFormat.Gif);
       
        //在页面上输出
        Response.Clear();
        Response.ContentType = "image/gif";
        Response.BinaryWrite(stream.ToArray());  
       
        stream.Close();
        stream.Dispose();
       
        graphics.Dispose();
        img.Dispose();
       
        Response.End();
    }

img.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Gif);
Response.ContentType = "image/gif";