创建缩略图

 string file = files[i];
                Image img = Image.FromFile(file);
                int width = img.Width;
                int height = img.Height;
                if (width > height)
                {
                    width = 800;
                    height = width * img.Height / img.Width;
                }
                else
                {
                    height = 800;
                    width = height * img.Width / img.Height;
                }
                Bitmap bt = new Bitmap(width, height);
                Graphics g = Graphics.FromImage(bt);
                g.DrawImage(img, 0, 0, width, height);
                //g.Save(
                //Image small = img.GetThumbnailImage(width, height, null, IntPtr.Zero);

                bt.Save(string.Format("{0}m{1}.jpg", topath, i.ToString("0000")), ImageFormat.Jpeg);

来源：https://www.cnblogs.com/gateluck/archive/2011/07/13/2105267.html