WikiPedia is a good source for images because all the sharing/use terms are explicitly listed for images. We have other image sources listed on our Image Resources page of our COV project Google Site.
The wiki site "Copyright-Friendly and Copyleft Images and Sound (Mostly!) for Use in Media Projects and Web Pages, Blogs, Wikis, etc." also has an extensive list worth checking out.
David Warlick has an extensive but perhaps overwhelming list of websites which can be used as image sources on his Landmarks for Schools site.
I used to teach a workshop titled "Multimedia Madness: Obtaining Images, Movies, and Audio Files from the Internet and CD-ROMs," and although the list is a a bit dated there are some image sources there that could be helpful.