I have been testing PNG settings with the two following lines.
invoke GdipBitmapConvertFormat,pclone,21808h,0,0,NULL,0 ; PixelFormat24bppRGB
or
invoke GdipBitmapConvertFormat,pclone,21005h,0,0,NULL,0 ; PixelFormat16bppRGB555
Even after a lot of testing I still cannot see any difference between the 24bpp and the 16bpp versions. The 5*5*5 version is a lot smaller and the quality looks just as good.