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","文字");

source:http://feisha.cn/Articles/Art.62.htm