新建一个24bit的bmp,把原image画上去var bmp2 = new Bitmap(bmp1.Width, bmp1.Height, PixelFormat.Format24bppRgb);
using (var g = Graphics.FromImage(bmp2))
g.DrawImage(bmp1, 0, 0, bmp2.Width, bmp2.Height);
bmp2.Save(..., ImageFormat.Bmp);
using (var g = Graphics.FromImage(bmp2))
g.DrawImage(bmp1, 0, 0, bmp2.Width, bmp2.Height);
bmp2.Save(..., ImageFormat.Bmp);
g.DrawImage(bmp1, 0, 0, bmp2.Width, bmp2.Height);