Thanks to all who replied. I knew about Preview, but it's just not
what I wanted. What I needed to remember was GraphicConverter. It's
been a long time since I was a serious Mac user, so thanks much for
jogging my memory. There were lots of commercial progs that worked
well, but this is for a friend who doesn't have a lot of money to
spend, so shareware/freeware is the way to go, and GraphicConverter
was the best of the lot, IIRC.