http://www.youtube.com/yt/copyright/...copyright.html
http://www.youtube.com/yt/copyright/fair-use.html
http://www.youtube.com/yt/copyright/...e-commons.html
I'll just leave this here, may be for videos, but I would say the basic outlines of rules for copyright apply well here.
If you are really worried about putting yourself on thin ice, you might want to do some research.