I'm making an ios app based on the Fallout series. Some of the artwork is straight from the Fallout games. I'm not charging for the app and there will be no adds, it's strictly a fan app. Can I get in trouble for this?

Yes, because you are republishing copyrighted assets. You need to get written permission from Bethesda to avoid any legal entanglements.

As long as it falls under the fair use policy you should be fine.