I'm sure a screen shot would fall under fair use. For example you're doing a review of a game, and want to show a few screen shots. That's what fair use is. However you do have rights to the files you prepare. Someone can not simply take them, put them on their site and claim fair use. You did the work. You control how the file is used.
A lot of people will let you use files if you give credit, but you have to remember that's exactly what is happening. They are letting you use them. You can't โforceโ them to let you use them by offering credit, cash, or anything else.