We don’t define Fair Use; a court does. Until something has actually gone to court, there are only the would-be plaintiff’s arguments why it doesn’t qualify, and the would-be defendant’s arguments why it does.
Images aren’t “special” with respect to Fair Use—they are copyrighted material, which means the copyright holder can control their use by others, e.g. via a license. Fair Use, however, refers to a narrow set of cases where one can use unlicensed copyrighted material in certain ways for certain purposes. What the copyrighted material is doesn’t matter, only that it is copyrighted and we don’t hold either the copyright or a license to use it. The same considerations governing all Fair Use govern the use of an image:
- the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
- the nature of the copyrighted work;
- the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
- the effect of the use upon the potential market for or value of the copyrighted work.
(17 U.S. Code § 107. Limitations on exclusive rights: Fair use)
(This applies for US law; other countries have, as I understand it, generally similar laws but details may vary.)
These are four things to consider, not four requirements—so just because we are educational doesn’t automatically mean everything we might do is Fair Use, but the fact that “the nature of the copyrighted work” here is undisputedly deserving of “strong” copyright protection doesn’t mean our use can’t be Fair Use, either. Ultimately, where to draw the line on a particular use is up to a judge, and we can only try to do our best to consider the same things the judge will.
As I have stated elsewhere, in what seems to be accepted policy:
Deciding whether or not something qualifies for Fair Use is a subjective process, that can basically only be performed by a judge, in a courtroom, when a suit is brought and Fair Use is brought up as a defense. None of these things is a firm “this is OK, that is not,” they are considerations that the law has directed judges to look at when thinking about potential Fair Use.
In the context of our site, we can look at these four considerations:
Our purpose is educational, and since Stack Exchange does not (currently) serve ads on this site (aside from the ones we make ourselves and that Stack Exchange receives no money for), profit is not really involved. Again, since there isn’t a firm line here, there being some tangential profit or potential future profit is not necessarily damning; newspapers can include quotations for reviews, which helps them sell newspapers (which make money because of the ads in them), and that is an explicit example of fair use.
The nature of the copyrighted works we deal with don’t do us any particular favors here; that’s primarily about limiting copyright protections from protecting data that isn’t really the author’s to protect. The rules text and/or diagrams, the art included in the books, and so on, are all very much the property of their respective owners. But again, these aren’t criteria for Fair Use, they are things to consider.
and 4. The last two are important on this site, though: how much of the work are you quoting, and how is that going to affect the value of the work? Where the first point is basically always in our favor, at least for legitimate answers, and the second is basically not, the majority of the time, meeting the last two is where we can say that one use is OK and another is not. The crucial test here is that your use is not fair if it replaces the original work: if there is no reason to buy the product or otherwise consume the work after seeing your use of it, you are violating the copyright holder’s rights.
In short, the court would likely look favorably on usages that limit themselves to just what they need to make their larger point (educational, in our case), and that don’t unduly affect the value of the original work in the process. Quoting some-but-not-all of the rules for a class, even quoting an entire spell’s or feat’s rules, including one of many of the images in a book, so long as they are justified by improving the educational value of the answer, these things are unlikely to be seen as violating copyright because the book still has so much more left in it of value—you aren’t threatening the original work with these kinds of things.
Ultimately, however, Fair Use is a subjective thing. As I said, it only really gets pinned down if you get sued and make the case in front of a judge; until then you just have your rationale, and the copyright holder may have a different one. Neither is definitively “right” or “wrong” until it gets tested in court. As a matter of practice, copyright infringements are dealt with by the copyright holder sending a cease & desist letter, and then, if that is ignored, possibly a lawsuit. The C&D always starts matters because it is vastly less expensive than a lawsuit.
As far as meta policy is concerned
The real question here is how we should view potential copyright infringement/fair use cases as a matter of policy on this site. Realistically, the risk we face is a C&D forcing us to remove the infringing material, forcing an answer to be reworked to go without it—obviously, we are/Stack Exchange is not going to ignore a C&D if we get one, it’s just not worth it.
Which has been the guiding principle for how this site handles copyright for a long time: we don’t link to pirating sites primarily for this reason. Sure, many of us also have philosophical objections to piracy, but the real point for this site is that infringing sites frequently get C&D’d out of existence, leading to link rot and bad answers. That is the primary thing to avoid as far as this site is concerned.
So any particular use case has to be considered in light of how likely is it to suffer due to a C&D causing a bad answer. That means we need to keep points 3. and 4. in mind when considering a potential use. That means that, if something is over the line, it’s a bad answer because of the risk of damage from a C&D.
And if that’s our policy, the risk of an actual lawsuit is just about negligible. C&D’s happen first and happen more often/more easily; if we are trying to avoid those, we’re all-but-certain to avoid lawsuits.
In this case, a single image from a book is probably fine. Exceptions are plausible if the image is somehow like the whole point of the book, but I can’t see that happening with RPG material. If the image was a separate product, that might not work so well.
Also, linking to something is not a question of copyright. Copyright can’t stop anyone from linking to anything. As a matter of policy, we don’t link to places that don’t have the rights to host something, since it’s liable to get taken down and thus the link rots, but there is never a concern for us to be guilty of copyright infringement for merely linking to something—even if that thing we’ve linked to is infringing upon copyright. The above is concerned solely with hosting and displaying contents ourselves.
Anyway, if we link to a site where an artist has chosen to host their own work, it is more than reasonable to assume that the artist has given that site a license to display the work. No one should ever be editing out such links under copyright concerns; that’s just a gross misunderstanding of how copyright works.