Excerpt from Code of Best Practices in Fair Use for Online Videos:
Fair use is flexible; it is not uncertain or unreliable. In fact, for any particular field of critical or creative activity, lawyers and judges consider expectations and practice in assessing what is “fair” within the field. In weighing the balance at the heart of fair use analysis, judges refer to four types of considerations mentioned in the law: the nature of the use, the nature of the work used, the extent of the use and its economic effect. This still leaves much room for interpretation, especially since the law is clear that these are not the only necessary considerations. In reviewing the history of fair use litigation, we find that judges return again and again to two key questions:
Both questions touch on, among other things, the question of whether the use will cause excessive economic harm to the copyright owner.
If the answers to these two questions are “yes,” a court is likely to find a use fair. Because that is true, such a use is unlikely to be challenged in the first place.
These sites contain collections of mostly still images. Some have moving images as well.
Landmark College Library has the best information I can find about citing images found on the web. The examples include how to cite something when there's not author or title associated with the image you're citing.