IMDb isn't the copyright holder of the original image. So they aren't really the ones to ask.
There is a clause in copyright law that allows for what's called "Fair Use". Meaning, you're allowed to use copyrighted material for educational, parody, newsworthy (comment, criticize, etc), non-commercial purposes (you can't make money from it). I'm not a lawyer and this is my own understanding of the law. Such use, for example, allows you and me to use our respective avatars without being sued.
