There is no clear yes/no answer to your question. Fair use is a fuzzy distinction. My understanding is that there are a variety of factors which will make a fair use defense more likely. If you are using the copyrighted material for something like criticism, if you are not making money, if you are not harming sales of the copyright holder, and if you are only using a small piece of the whole (or perhaps a small representation of an image).
And so you are most likely safe using a small image of a poster in a movie review on a non-money making blog (or low money-making). However, selling full size posters, wouldn't be acceptable
For your situation, probably the worst that would happen to you is you would get a Cease and Desist letter form the copyright holder, asking you to remove the content. Getting sued is unlikely.