For example, in this question about a scene in the film Metropolis, is it allowed to copy the image from a web site and put it directly in the question? It's my understanding it breaks copyright but I'm not very knowledgeable about how that works.
The 100% legal answer is complicated because it dives into the murky jungle of copyright law and the fair use defense, but the answer is "should be OK as long as you don't overdo it."
For a real, legally valid opinion on this question you'd have to ask SE to ask a lawyer, which I am not. However, in layman's terms, what we do with images from movies is probably not going to get anyone in trouble, because it probably falls under the fair use defense. The tricky part about fair use is that it's not very clearly defined: the only sure way to know if something qualifies as fair use is to ask a jury -- at your trial.
But there are some guidelines for what constitutes fair use of a copyrighted work, and Stack Exchange tends to tick off a lot of those boxes:
- Only use very small, selected excerpts from the original ("as little as necessary")
- Using "lower quality" versions of the original (for example, use still images instead of a whole video)
- Not using the excerpts for commercial purposes
- Using the excerpts for commentary or criticism.
Fair use was basically invented to allow the kind of uses we have for copyrighted material: to allow us to discuss a work, and use examples, in order to critique it, or explain it, or comment on it.
Since what we're doing is probably fair use, chances are any copyright holders that do stumble on our site are not going to waste their very expensive lawyer's time trying to sue Stack Exchange or issue DMCA notices or anything, because those lawyers would explain that they would probably lose.
(As I understand it, SO is fully protected by the DMCA here, because it's user-provided content, so the first we'd know about it is the site getting a takedown notice, which AFAIK has never happened here. I would definitely hope if such a thing occurred the CMs would lets us know to cut it out.)
Disclaimers: IANAL (I am not a lawyer), nor do I represent SE in any way (I'm a volunteer moderator on this site and do not speak for the company).
From the Stack Exchange Network Content Policy:
Copyright. Using copyrighted material does not constitute infringement in all cases. In general, however, users should be careful when using copyrighted content without the permission of those who created it. It is our policy to respond to notices of alleged infringement that comply with the Digital Millennium Copyright Act ("DMCA").
The issue of "safely adding images" falls primarily under the clauses of "Fair Use" of US Copyright law. The concept of fair use permits limited use of copyrighted material for things like teaching, reporting, and criticism. That mean if (for example) you are writing a critical analysis of a book, you are allowed to show the cover of that book to help describe what you are talking about, or to cite a short passage from that book to illustrate a point.
But almost all fair use cases describe a general rule that no more of the copy or trademark work should be used than is necessary for the legitimate purpose. What that means, essentially, is that we should try to work within the bounds of common sense. For the purpose of operating your site (without getting into a bunch of legal hassles), you should concern yourself primarily with editing for valid and relevant content. If a photo or movie clip does not substantially add to the post, it should be removed. If the content is obviously infringing, it should be removed.
Finally, I should note that there are already many many posts on SFF which contain screenshots, gifs, or video clips taken from films, and as far as I know we've never had a DMCA takedown notice yet.
In your specific case, we're only talking about a single image from the film, which is clearly relevant to the topic at hand and substantially improves the question, making it much quicker and easier for people to understand what you're asking about. I doubt that this could possibly be an issue.
TL;DR: it's fine.
1 Hat-tip to @Loong for drawing my attention to this post.
Fair use regarding film excerpts for criticism and review purposes is usually interpreted as max. 2 minutes. Studios themselves usually come up with max. 4 min excerpts for press usage.
Some owners dislike stills revealing the ending, which should be fine to use after the movie had its theatrical run already. Common sense.