The best cross-platform file format wold be tiff or jpg at "best quality". The worst (IMHO) would be bum. There is a shareware app available for MAC OS called GraphicConverter which can translate almost any graphic file. It has over 100 different parsers including many obscure ones (I.E., Atari & TRS 80 formats). The only ones it can't work with are some "native" formats like Painter riff or Corel paint.