using System.Drawing;
using System.IO;
using System.Drawing.Imaging;

        private void AddTextToImg(string fileName,string text)
        {
            if(!File.Exists(MapPath(fileName)))
            {
                throw new FileNotFoundException("The file don\'t exist!");
            }
           
            if( text == string.Empty )
            {
                return;
            }
            //还需要判断文件类型是否为图像类型，这里就不赘述了

            System.Drawing.Image image = System.Drawing.Image.FromFile(MapPath(fileName));
            Bitmap bitmap = new Bitmap(image,image.Width,image.Height);
            Graphics g = Graphics.FromImage(bitmap);

            float fontSize = 12.0f;             //字体大小
            float textWidth = text.Length*fontSize;  //文本的长度
            //下面定义一个矩形区域，以后在这个矩形里画上白底黑字
            float rectX = 0;       
            float rectY = 0;
            float rectWidth = text.Length*(fontSize+8);
            float rectHeight = fontSize+8;
            //声明矩形域
            RectangleF textArea = new RectangleF(rectX,rectY,rectWidth,rectHeight);

            Font font = new Font("宋体",fontSize);   //定义字体
            Brush whiteBrush = new SolidBrush(Color.White);   //白笔刷，画文字用
            Brush blackBrush = new SolidBrush(Color.Black);   //黑笔刷，画背景用

            g.FillRectangle(blackBrush,rectX,rectY,rectWidth,rectHeight);           

            g.DrawString(text,font,whiteBrush,textArea);
            MemoryStream ms = new MemoryStream( );
            //保存为Jpg类型
            bitmap.Save(ms,ImageFormat.Jpeg);

            //输出处理后的图像，这里为了演示方便，我将图片显示在页面中了
            Response.Clear();
            Response.ContentType = "image/jpeg";
            Response.BinaryWrite( ms.ToArray() );

            g.Dispose();
            bitmap.Dispose();
            image.Dispose();
        }

调用时很简单，

AddTextToImg("me.jpg","小李");

一切OK了，感觉.net确实好强大，这些功能在Asp中可是奢侈品了，而在.Net环境中却能轻而易举的完成！