It's not even always illegal to use part of someone else's work. Acts of parody or satire, reporting, education, or (sometimes) use of a small part of something else are all generally considered protected. So, for example, Weird Al could get away with making his parodies without permission (even though he makes money from them; and, he actually does get permission, because that's the kind of guy he is). TV news stations don't have to pay royalties if they cover an event at which some kids are singing "Happy Birthday", though the exact same footage shot as part of a movie would require royalties and permission. I could show my classroom a map of Middle Earth if it were relevant to, say, a unit on Tolkein. And Google can put up image thumbnails on its search pages without getting busted.
The usual five-letter acronym applies; this is just my somewhat-informed understanding of things. Pasha, legally able to leave out the N from the acronym, could maybe do better.
