参考：https://www.cnblogs.com/ZXdeveloper/p/6841878.html

由于参考的博文提供的代码在我的pc上跑起来，会使压缩后的图片有白色边框，所以我做了小小的修改

        /// <summary>
        /// 无损压缩图片
        /// </summary>
        /// <param name="sFile">原图片地址</param>
        /// <param name="dFile">压缩后保存图片地址</param>
        /// <param name="flag">压缩质量（数字越小压缩率越高）1-100</param>
        /// <param name="size">压缩后图片的最大大小</param>
        /// <param name="sfsc">是否是第一次调用</param>
        public static bool CompressImage(string sFile, string dFile, int flag = 90, int size = 300, bool sfsc = true)
        {
            //如果是第一次调用，原始图像的大小小于要压缩的大小，则直接复制文件，并且返回true
            FileInfo firstFileInfo = new FileInfo(sFile);
            if (sfsc == true && firstFileInfo.Length < size * 1024)
            {
                firstFileInfo.CopyTo(dFile);
                return true;
            }
            Image iSource = Image.FromFile(sFile);
            ImageFormat tFormat = iSource.RawFormat;

            Bitmap ob = new Bitmap(iSource.Width, iSource.Height);

            Graphics g = Graphics.FromImage(ob);
            g.Clear(Color.White);
            g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            g.DrawImage(iSource, new Rectangle(0, 0, iSource.Width, iSource.Height), 0, 0, iSource.Width, iSource.Height, GraphicsUnit.Pixel);
            g.Dispose();

            //设置压缩质量
            EncoderParameters ep = new EncoderParameters();
            ep.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, new long[1] { flag });

            try
            {
                //找到系统中可用的图片编码器信息
                ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();
                ImageCodecInfo jpegICIinfo = null;
                for (int x = 0; x < arrayICI.Length; x++)
                {
                    if (arrayICI[x].FormatDescription.Equals("JPEG"))
                    {
                        jpegICIinfo = arrayICI[x];
                        break;
                    }
                }
                //如果编码器存在的，可以压缩
                if (jpegICIinfo != null)
                {
                    ob.Save(dFile, jpegICIinfo, ep);
                    FileInfo fi = new FileInfo(dFile);
                    if (fi.Length > 1024 * size)
                    {
                        flag -= 10;
                        CompressImage(sFile, dFile, flag, size, false);
                    }
                }
                else
                {
                    ob.Save(dFile, tFormat);
                }
                return true;
            }
            catch
            {
                return false;
            }
            finally
            {
                iSource.Dispose();
                ob.Dispose();
            }
        }

调试代码：

string spath = @"G:\Backup\pq1.jpg";
string dpath = @"G:\Backup\pq3.jpg";
CompressImage(spath,dpath);

结果：压缩前（1.52 MB）

压缩后（268 KB）