everything you make is your copyright. and the buyer has no say unless they bought the right to keep that copyright. then its theirs. now if its a picture of them - i doubt it would sell. not sure how it works if its a painting of a photo they gave you.
but generally if you sell the original to someone - that copy is yours. be sure you have a very good copy of that image before you sell it though.
When you sell an original, they only buy THAT WORK, not the copyright. You can always make prints of your works when you still own the copyright. I even believe that for past sold pieces, that the buyer MUST allow access to the artist for taking a photograph of the piece if you request to...but I am not positive about that. And if an original painting is sold, the artist can also paint any similar piece to it again...for the artist still owns the copyright. Copyright is not accidentally sold with the piece, and is worth much more than the price of any original.
you shouldn't be making famous people at all. those people or the family there of, can sue you. it has to do with the fame of a face. and if you took it from a photo - the photographer can go after you.
I could take a photo of a best impersonator of Elvis or other famous people? I know there are many festivals and many impersonators are out and people are taking photos.
Anyway I need to check that out.
i suppose you could, i don't know that totally works. you'll have to check the local laws, both where you actually live and where you sort of currently live. i think in canada there is a law where you can't sell a person as art (as far as i know), if they are the only one in the scene. i think they have to be in a group or be a few of them. i adopted that for american shots just to be safe. not sure how it applies to other kinds of art.