The PICT file format is documented in Appendix A of " Inside Macintosh: Imaging with QuickDraw," which is available on Apple's Developer World web site.
Apple does not recommend reading or writing PICTs directly -- instead,
use the usual documented QuickDraw routines (e.g. OpenCPicture,
ClosePicture, DrawPicture). For example, if you wanted to
parse a PICT for PicComments, you could define a commentProc and patch out the
QuickDraw bottlenecks. This is guaranteed to work, while parsing PICTs
directly requires writing and testing a fair amount of code, and is likely to
break in the future.