The phrase "fair use" is shorthand for the circumstances which permit use of copyrighted works, or which place limitations on the exclusive rights of copyright holders. In the United States, 17 U.S. Code§ 107 states that "the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright."
In order to determine whether the use of copyrighted materials is fair, courts weigh four factors that the law circumscribes (see the next section). The weighing of these factors, however, is extremely tricky and seemingly subjective, which makes many individuals and institutions wary of using copyrighted materials in their work.
Does [insert highly specific situation here] qualify as fair use?
That's a tough question! Fair use is incredibly complicated, and despite having been around for a long time, continues to be argued in courts. Often there are compelling arguments to be made defending fair use and copyright infringement claims which have to be weighed against each other, and within the context of purpose of copyright (protecting intellectual property holders).
So not only is that a question that (possibly) doesn't have a straightforward answer, we're not qualified to try to give you one. But we're happy to provide explanation about the four factors to help you make an informed decision regarding whether to appropriate copyrighted works, or help you find images that are licensed for reuse!