代码如下:
/// <summary>
        /// 压缩图片
        /// </summary>
        /// <returns></returns>
        public string ResizePic()
        {
            #region 压缩图片开始
            bool IsImgFile = true;  //判断是否为图片文件
            string filePathName = "123";   //文件存储的路径（文件夹名称）
            string fileName = "a.jpg";   //上传文件的原始名称
            string fileSysName = DateTime.Now.ToString("yyyyMMddHHmmssfff") + "_" + fileName;  //修改后的文件名称
            string filePath = "";   //文件路径
            string strImgPath = "/fileupload/";   //上传路径
            if (IsImgFile)
            {
                int maxWidth = 600;   //图片宽度最大
                int maxHeight = 400;  //图片高度最大
                System.Drawing.Image imgPhoto =
                    System.Drawing.Image.FromFile(Server.MapPath(strImgPath) + filePathName + "/" + fileSysName);
                int imgWidth = imgPhoto.Width;
                int imgHeight = imgPhoto.Height;
                if (imgWidth > imgHeight)  //如果宽度超过高度以宽度为准来压缩
                {
                    if (imgWidth > maxWidth)  //如果图片宽度超过
                    {
                        float toImgWidth = maxWidth;   //图片压缩后的宽度
                        float toImgHeight = imgHeight / (float)(imgWidth / toImgWidth); //图片压缩后的高度

                        System.Drawing.Bitmap img = new System.Drawing.Bitmap(imgPhoto,
                                                                              int.Parse(toImgWidth.ToString()),
                                                                              int.Parse(toImgHeight.ToString()));
                        string strResizePicName = Server.MapPath(strImgPath) + filePathName + "/_small_" + fileSysName;
                        img.Save(strResizePicName);  //保存压缩后的图片
                        filePath = strImgPath + filePathName + "/_small_" + fileSysName;  //返回压缩后的图片路径
                    }
                }
                else
                {
                    if (imgHeight > maxHeight)
                    {
                        float toImgHeight1 = maxHeight;
                        float toImgWidth1 = imgWidth / (float)(imgHeight / toImgHeight1);

                        System.Drawing.Bitmap img = new System.Drawing.Bitmap(imgPhoto,
                                                                              int.Parse(toImgWidth1.ToString()),
                                                                              int.Parse(toImgHeight1.ToString()));
                        string strResizePicName = Server.MapPath(strImgPath) + filePathName + "/_small_" + fileSysName;
                        img.Save(strResizePicName);
                        filePath = strImgPath + filePathName + "/_small_" + fileSysName;
                    }
                }
            }
            return filePath;
            #endregion
        }