The problem of copyrighted images and fair used recently arose here: What does the final て in 待ってて signify?
That's the first place I've seen this issue arise, but that's probably not the first case of having a copyrighted image posted to SE. I myself used an illustration from a dictionary once, and I've seen other helpful illustrations and diagrams being posted. Sooner or later, we'll need a policy for such cases.
I would like to think that was all legal, and I would hate to lose all these nice images, but it's not for me to decide. It should probably be decided by Stack Exchange, really, since they are the ones facing litigation if anything goes wrong, but I just don't know what is SE's policy about that - if they have one.
I guess US law applies here, which means fair use, safe harbor and cease and desist letters. This all sounds quite lenient to me, but IANAL, and I know that at least fair use is a very tricky subject with no defined guidelines. So I'd feel more comfortable if I have a clear (or even semi-clear) answer about that.