For a media class that I teach occasionally we are often looking for media that students can use to create videos and such. To model digital citizenship, we endeavour to use media that is at least free (gratis), but preferably public domain or creative commons. There are, of course, some creative commons licences that allow sharing but don't allow derivatives, which means that they are not useful for us.
There are a few caveats about using creative commons or public domain works, though. Since students don't tend to be as familiar with the works/artists it often takes them longer to complete their projects than if you just let them bring in their own (usually quasi-legal) media. As well, the onus is usually on you to point the students in the right direction.
To help with this, a few sources of media that I've come across are:
Archive.org (also has other types of files)
100 (Legal) Sources for Free Stock Images
and, of course, Flickr
Free Vector Graphics
Quality Vector Graphics