自定义一个处理图片的HttpHandler

  public class ImageHandler : IHttpHandler
    {
        #region IHttpHandler Members

        public bool IsReusable
        {
            get { return true; }
        }

        /// <summary>
        /// 实现IHTTPHandler后必须实现的方法
        /// </summary>
        /// <param name="context">HttpContext上下文</param>
        public void ProcessRequest(HttpContext context)
        {
            context.Response.Clear();
            //得到图片名
            var imageName = context.Request["ImageName"] == null ? "逆时针的风"
                : context.Request["ImageName"].ToString();
            //得到图片ID,这里只是演示，实际项目中不是这么做的
            var id = context.Request["Id"] == null ? "01"
                : context.Request["Id"].ToString();
            //得到图片地址
            var stringFilePath = context.Server.MapPath(string.Format("~/Image/{0}{1}.jpg", imageName, id));
            //声明一个FileStream用来将图片暂时放入流中
            FileStream stream = new FileStream(stringFilePath, FileMode.Open);
            using (stream)
            {
                //透过GetImageFromStream方法将图片放入byte数组中
                byte[] imageBytes = this.GetImageFromStream(stream,context);
                //上下文确定写到客户短时的文件类型
                context.Response.ContentType = "image/jpeg";
                //上下文将imageBytes中的数据写到前段
                context.Response.BinaryWrite(imageBytes);
                stream.Close();
            }
        }

        /// <summary>
        /// 将流中的图片信息放入byte数组后返回该数组
        /// </summary>
        /// <param name="stream">文件流</param>
        /// <param name="context">上下文</param>
        /// <returns></returns>
        private byte[] GetImageFromStream(FileStream stream, HttpContext context)
        {
            //通过stream得到Image
            Image image = Image.FromStream(stream);
            //加上水印
            image = SetWaterImage(image, context);
            //得到一个ms对象
            MemoryStream ms = new MemoryStream();
            using (ms)
            {
               //将图片保存至内存流
                image.Save(ms, ImageFormat.Jpeg);
                byte[] imageBytes = new byte[ms.Length];
                ms.Position = 0;
                //通过内存流读取到imageBytes
                ms.Read(imageBytes, 0, imageBytes.Length);
                ms.Close();
                //返回imageBytes
                return imageBytes;
            }
        }
        /// <summary>
        /// 为图片加上水印，这个方法不用在意，只是演示，所以没加透明度
        /// 下次再加上吧
        /// </summary>
        /// <param name="image">需要加水印的图片</param>
        /// <param name="context">上下文</param>
        /// <returns></returns>
        private Image SetWaterImage(Image image,HttpContext context) 
        {
            Graphics graphics = Graphics.FromImage(image);
            Image waterImage = Image.FromFile(context.Server.MapPath("~/Image/逆时针的风01.jpg"));
            //在大图右下角画上水印图就行
            graphics.DrawImage(waterImage,
                new Point { 
                    X = image.Size.Width - waterImage.Size.Width,
                    Y = image.Size.Height - waterImage.Size.Height 
                });
            return image;
        }

        #endregion
    }

    <httpHandlers>
      <add type="ImageHandler.ImageHandler,ImageHandler"  verb="*" path="ImageHandler.apsx"/>
    </httpHandlers>

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <h2>
        About
    </h2>
    <p>
        Put content here.
        <asp:Image runat="server" ImageUrl="ImageHandler.apsx?ImageName=逆时针的风&Id=02" />
    </p>
</asp:Content>