private void saveJpeg(string path, Bitmap img, long quality)
{
   EncoderParameter qualityParam =
      new EncoderParameter(Encoder.Quality, (long)quality);

   ImageCodecInfo jpegCodec =
      this.getEncoderInfo("image/jpeg");

   if(jpegCodec == null)
      return;

   EncoderParameters encoderParams = new EncoderParameters(1);
   encoderParams.Param[0] = qualityParam;

   img.Save(path, jpegCodec, encoderParams);
}

private ImageCodecInfo getEncoderInfo(string mimeType)
{
   ImageCodecInfo[] codecs = ImageCodecInfo.GetImageEncoders();

   for (int i = 0; i <codecs.Length; i++)
      if (codecs[i].MimeType == mimeType)
         return codecs[i];
   return null;
}

Image img = Image.FromFile(imagePath);
img.Save(pathToSave, System.Drawing.Imaging.ImageFormat.Jpeg);

        private void button2_Click(object sender, EventArgs e)
        {
            string sDirectory;
            FolderBrowserDialog FolderBrowserDialog1 = new FolderBrowserDialog();
            {                
                FolderBrowserDialog1.RootFolder = Environment.SpecialFolder.Desktop;
                FolderBrowserDialog1.SelectedPath = "C:\\";
                FolderBrowserDialog1.Description = "Select folder with the desired files to process:";
                if (FolderBrowserDialog1.ShowDialog() == DialogResult.OK)
                {
                    sDirectory = FolderBrowserDialog1.SelectedPath;
                    ProcessDir(sDirectory);
                }
            }
        }
        public void ProcessDir(string sDir)
        {
            int x = 0;
            int k;
            string sExtension;
            string[] fileEntries = Directory.GetFiles(sDir);

                foreach (string fileName in fileEntries)
                {
                    x++;
                    k = fileName.Length;
                    sExtension = null;
                    for (int j = 3; j != 0; j--)
                    {
                        sExtension = sExtension + Convert.ToString(fileName[k - j]);
                    }
                    if (sExtension == "bmp")
                    {

                        Image img = Image.FromFile(fileName);
                        Directory.CreateDirectory(sDir + "\\Processed");
                        img.Save(sDir + "\\Processed\\img" + x + ".jpg", System.Drawing.Imaging.ImageFormat.Jpeg);
                    }
                }
        }

    }