PICT file is an object oriented native Macintosh file format. A BitMap is one possible object in a PICT file (other object include: Text, BitMaps, lines, circles, regions, fill patterns, etc..). All programs that use, graphics on the Macintosh can use PICT files, but not all program can edit, objects in the PICT files, some programs convert them to BitMaps.
PICT2 - Extension to PICT file format to include Color information.
PNGT - MacPaint file type - 72 DPI, 576 by 720 dots B&W BitMap .
SCRN - Macintosh StartUp screen 512 by 342 B&W BitMap.