Having explored this issue quite a bit running a media company, I wholeheartedly concur with Shel. If you use copyrighted celebrity images (especially if they come from Getty or another major stock provider), they will come after you. You could try to argue fair use at that point, but you will spend thousands or tens of thousands in legal fees to fight your case, even if you're in the right (and you're probably not).
A couple of suggestions. Per Shel, write their agent/visit website for approved images (you can get contact information for celebs via IMDB Pro - not that expensive). Second, look up publicity info/sites for movies they have been in. George Clooney's image may not be cheap on Getty, but maybe there are publicity images of Clooney from the Ocean's Twelve website that you could use.