Okay. I just found an article which gave me the answer I'm looking for. Photographs may be used as source material if your painting transforms the final image so that the source photograph is no longer recognizable in a side-by-side comparison. And paintings may be created of famous people if the painting itself has artistic merits that go beyond a mere realistic representation of the person depicted. That's why Warhol's paintings are protected, but a realism portrait of Marilyn Monroe might not be.
The goal of the courts, it seems is to determine whether the artwork has a goal or artistic message apart from a mere representation of a certain celebrity. If the art is making a social commentary or is depicted in a non-realistic style then it is apparently protected. If it's just a painting of a realistic image of a celebrity, then it's possibly not protected because the courts may see the art as merely trying to capitalize on the celebrity of a certain individual without adding anything to the work.
Looks like portraits in my own "pop" style would be fine as long as I make sure they don't look just like a photo, which they don't because that's not what I do!