[원문]
http://stackoverflow.com/questions/4161873/reduce-image-size-c-sharp

public static void SaveJpeg(string path, Image img, int quality)
        {
            if (quality < 0 || quality > 100)
                throw new ArgumentOutOfRangeException("quality must be between 0 and 100.");

            // Encoder parameter for image quality
            EncoderParameter qualityParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
            // JPEG image codec
            ImageCodecInfo jpegCodec = GetEncoderInfo("image/jpeg");
            EncoderParameters encoderParams = new EncoderParameters(1);
            encoderParams.Param[0] = qualityParam;
            img.Save(path, jpegCodec, encoderParams);
        }

        /// <summary>
        /// Returns the image codec with the given mime type
        /// </summary>
        private static ImageCodecInfo GetEncoderInfo(string mimeType)
        {
            // Get image codecs for all image formats
            ImageCodecInfo[] codecs = ImageCodecInfo.GetImageEncoders();

            // Find the correct image codec
            for (int i = 0; i < codecs.Length; i++)
                if (codecs[i].MimeType == mimeType)
                    return codecs[i];

            return null;
        }