GraphicConverter is a popular option used by many for converting from MacPaint to PICT. But the ne ultra? PICTure This, it runs well on classic Macintoshes with only 1 MB of memory, is very memory efficient, and can run on System 6.
https://68kmla.org/forums/index.php?app=forums&module=forums&controller=topic&id=60485
https://macintoshgarden.org/apps/picture-20