System.Drawing.Image newimage = System.Drawing.Image.FromFile("c:\\00.jpg");
Bitmap bt = new Bitmap(newimage);
Graphics g = Graphics.FromImage(bt);

（中间略去在画布上绘图的代码）

string lujing = @"c:\1.jpg";

//处理JPG质量的函数
int level = 100; //图像质量 1-100的范围
ImageCodecInfo[] codecs = ImageCodecInfo.GetImageEncoders();
ImageCodecInfo ici = null;
foreach (ImageCodecInfo codec in codecs)
{
   if (codec.MimeType == "image/jpeg")
   ici = codec;
}
EncoderParameters ep = new EncoderParameters();
ep.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, (long)level);

bt.Save(lujing, ici, ep);

//释放位图缓存
bt.Dispose();