var image = Bitmap.FromFile( sourceFileName );

var jpgEncoder = ImageCodecInfo.GetImageDecoders().Single(codec => codec.FormatID == ImageFormat.Jpeg.Guid);

var myEncoder = System.Drawing.Imaging.Encoder.Quality;
var myEncoderParameters = new EncoderParameters(1);

var myEncoderParameter = new EncoderParameter(myEncoder, 100L);
myEncoderParameters.Param[0] = myEncoderParameter;

image.Save(resultFileName, jpgEncoder, myEncoderParameters);