Image类的保存问题

对 filename 指定的图像文件按指定大小修改再保存，image对象已释放，仍却出错
public void imageSizeChange(string filename)
{
   if(!System.IO.File.Exists(filename))return;
System.Drawing.Image thumbnail = new Bitmap(640, 335);
System.Drawing.Graphics graphic = ystem.Drawing.Graphics.FromImage( thumbnail );               

   graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
   graphic.SmoothingMode = SmoothingMode.HighQuality;
   graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
   graphic.CompositingQuality = CompositingQuality.HighQuality;

   System.Drawing.Imaging.ImageCodecInfo [] Info = System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders();    
   System.Drawing.Imaging.EncoderParameters Params = new System.Drawing.Imaging.EncoderParameters(1);
   Params.Param[0] = new EncoderParameter(Encoder.Quality, 100L);

   System.Drawing.Image image = new System.Drawing.Bitmap(filename);
   graphic.DrawImage(image, 0, 0, 640,335);
   image.Dispose();//这句已经释了 image 了啊
   thumbnail.Save(filename,Info[1],Params);  //怎么以同名文件保存时还是会出错呢
}

补充：.NET技术 ,  C#