Forum Moderators: not2easy
One way to try to get a handle on this is to think about it in parallel to the type of fair use you'd have in a book review. If you're discussing a book, fair use allows you to take short quotes from it to illustrate points you're making. But it doesn't allow you to use big blocks of text. And it also doesn't allow you to simply print text from the book without your own commentary/analysis applied to it. (Besides book reviews, this would also apply to short attributed quotes in essays and articles.)
Some people think that if they're not making money from the text/images they're using, it's fair use no matter what. But that's only one of the elements looked at. Also involved is how much of the longer work you're using, and whether your use of it is part of your own commentary/review/analysis. The latter would mean that the specific screenshot(s) used would need to be relevant to what you're saying, just as a quote in a book review would need to be relevant to the specific point you're making about the book. OTOH, if you are making/planning to make money from use of the images (affiliate links, adsense, selling items), the other elements of fair use have to be even stronger -- use even less of the work, be even more careful to make it only a small part of your commentary.
If possible, of course, you can always ask for permission from the copyright holder. That's easier if you're using frames from one movie, or one studio's movies; more difficult if it's a broad range of works. Getting the copyright holder's permission not only lets you use the work without having to worry about it, but also gives you a nice bit of cache in being able to say, "Used by permission..." which not many site owners can do.
Maybe I can find some public photos of some movie that I can use. I will check out some movie sites.