//参数： 原始图片，保存路径，压缩比

private void CompressImage(MagickNet.Image img,string toPath, long ratio)

{

     using (Bitmap thumbImage = new Bitmap(MagickNet.Image.ToBitmap(img)))

     {

           ImageCodecInfo jgpEncoder = GetEncoder(ImageFormat.Jpeg);

           System.Drawing.Imaging.Encoder myEncoder = System.Drawing.Imaging.Encoder.Quality;

           EncoderParameters myEncoderParameters = new EncoderParameters();

           EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, ratio);

           myEncoderParameters.Param[] = myEncoderParameter;

           thumbImage.Save(toPath, jgpEncoder, myEncoderParameters);

       }

 }

private ImageCodecInfo GetEncoder(ImageFormat format)

 {

       ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();

       foreach (ImageCodecInfo codec in codecs)

       {

           if (codec.FormatID == format.Guid)

           {

                return codec;

           }

            }

        return null;

 }

CompressImage(img, toPath, 90L);