I seem to recall someone answering a similar question not long ago (though you'll doubtless want to double-check it) who said that movie images, covers, etc. that are properly credited fall under "fair use" in a movie review. Personally, since I'm not certain on it, I generally look for public-domain pictures that are similar to things from the movie. I wonder if there's a way to contact someone at like IMDB with this question? Seems like they ought to know.