public class ImageSY : IHttpHandler
{
  public ImageSY()
  {
    //
    //TODO: 在此处添加构造函数逻辑
    //
  }
  #region IHttpHandler 成员
  public bool IsReusable
  {
    get { return true; }
  }
  public void ProcessRequest(HttpContext context)
  {
    //获得请求的物理图片路径
    string imagePath = context.Request.PhysicalPath;
    System.Drawing.Image image = null;
    if (File.Exists(imagePath))
    {
      //定义水印文字
      string text = "本图片来至我的网站";
      //定义水印文字字体大小
      int fontSize = 22;
      //水印文字字体
      Font font = new Font("宋体", fontSize);
      //根据图片物理地址加载图片
      image = System.Drawing.Image.FromFile(imagePath);
      Graphics g = Graphics.FromImage(image);
      //获取要绘制水印文字所需要的显示区域大小
      SizeF size = g.MeasureString(text, font);
      if (size.Width > image.Width || size.Height > image.Height)
      {
      }
      else
      {
        Brush brush = Brushes.Red;
        g.DrawString(text, font, brush, image.Width - size.Width, image.Height - size.Height);
        g.Dispose();
      }
    }
    else
    {
    }
    image.Save(context.Response.OutputStream, ImageFormat.Jpeg);
  }
  #endregion
}

<location path="images">
 <system.web>
  <httpHandlers>
  <!---对jpg文件添加水印-->
  <add verb="*" type="ImageSY" path="*.jpg"/>
  <add verb="*" type="ImageSY" path="*.gif"/>
  <add verb="*" type="ImageSY" path="*.bmp"/>
  </httpHandlers>
 </system.web>
 </location>