直接上代码：

    /// <summary>
    /// 根据指定压缩比率压缩图片
    /// </summary>
    /// <param name="original"></param>
    /// <param name="imagePath"></param>
    /// <param name="quality"></param>
    public static void CompressToQuality(Image original, String imagePath, int quality)
    {
        ImageCodecInfo jpgEncoder = null;
        ImageCodecInfo[] codecs = ImageCodecInfo.GetImageEncoders();
        foreach (ImageCodecInfo codec in codecs)
        {
            if (codec.FormatID == ImageFormat.Jpeg.Guid)
            {
                jpgEncoder = codec;
                break;
            }
        }
        if (jpgEncoder != null)
        {
            System.Drawing.Imaging.Encoder encoder = System.Drawing.Imaging.Encoder.Quality;
            EncoderParameters encoderParameters = new EncoderParameters(1);

            EncoderParameter encoderParameter = new EncoderParameter(encoder, quality);
            encoderParameters.Param[0] = encoderParameter;

            FileStream ms = new FileStream(imagePath, FileMode.Create, FileAccess.Write);
            original.Save(ms, jpgEncoder, encoderParameters);
            ms.Flush();
            ms.Close();
        }
    }