Here’s a helpful article on the concept of fair use,over at the Chicago Artist Resource site: http://www.chicagoartistsresource.org/articles/law-artists-fair-use

And while we’re thinking about legal issues,this post, Protecting Your Art Images Online, covers some excellent territory:http://www.creativetexturetools.com/news/2013/06/21/protecting-your-art-images-online