pixel format flags??

[Ceset]

well i made a reserach but couldnt find anything

here is the link
http://wiki.libsdl.org/SDL_ConvertSurfaceFormat#SDL_ConvertSurfaceFormat-1

i saw pixel format flags but how can i know what format my 24 bit .png file uses and where can i learn which format uses which flag.

or maybe i just misunderstood everything??