Bitmap bitmap = new Bitmap("input.jpg");
bitmap.Save("output.jpg", ImageFormat.Jpeg, 
new EncoderParameters(1, new EncoderParameter(Encoder.Quality, 50L)));

/** <summary>
        /// 保存JPG時(shí)用
        /// </summary>
        /// <param name="mimeType"></param>
        /// <returns>得到指定mimeType的ImageCodecInfo</returns>
        private static ImageCodecInfo GetCodecInfo(string mimeType)
        {
            ImageCodecInfo[] CodecInfo = ImageCodecInfo.GetImageEncoders();
            foreach (ImageCodecInfo ici in CodecInfo)
            {
                if (ici.MimeType == mimeType) return ici;
            }
            return null;
        }
        /** <summary>
        /// 保存為JPEG格式，支持壓縮質(zhì)量選項(xiàng)
        /// </summary>
        /// <param name="bmp"></param>
        /// <param name="FileName"></param>
        /// <param name="Qty"></param>
        /// <returns></returns>
        public static bool KiSaveAsJPEG(Bitmap bmp, string FileName, int Qty)
        {
            try
            {
                EncoderParameter p;
                EncoderParameters ps;
                ps = new EncoderParameters(1);
                p = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, Qty);
                ps.Param[0] = p;
                bmp.Save(FileName, GetCodecInfo("image/jpeg"), ps);
                
                return true;
            }
            catch
            {
                return false;
            }
        }

using System.Drawing;  
using ImageSharp;  
using System.IO;  
  
public void SaveImageWithQuality(Bitmap bmp, string path, int quality)  
{  
    using (var img = Image.FromBitmap(bmp))  
    {  
        img.Mutate(x => x  
             .Format(ImageFormat.Jpeg)  
             .Quality(quality)  
             .Save(path));  
    }  
}