The PICT format was a proprietary thing for Mac only. Nowadays, OS X takes snaps as TIFFs, but can also do JPG, BMP, GIF, what have you.
Back to the original question, I THINK an old program called JPEGview allowed you to open PICTs and save then as JPG's. I can't remember for sure if it could do that, but it was definitely System 7.1 friendly.