Yes. The graphic formats used by Doom are actually well known and documented on the Doom wiki:http://www.doomwiki.org/wiki/Picture_formathttp://www.doomwiki.org/wiki/Flat
I think someone said that pngs are palette removing which I think would thus make the graphic work with Doom, Heretic, Hexen, Strife, ect.... Right?
More precisely, PNG files can be paletted or in true color (24-bit RGB, or 32-bit RGBA), but a paletted PNG file contains its own palette instead of being dependent on an external palette like Doom's native graphic formats.
Then ZDoom is able to match the colors to their closest approximation in the palette it uses, so you don't have to do the conversion yourself. And GZDoom is free from the palette entirely.