PICT format
I've written PICT bitmap decoders for non-Mac platforms. PackBits uses a form
of run-length encoding that's used in MacPaint pictures, for example… and you
should be able to find code for that in many places on CompuServe's libraries.
In short, if bytecode's high bit is set, then repeat the next byte
(256-bytecode)'s times.
What sort of translator are you writing?