The Math Images Project
The Math Forum is a comprehensive site with many different resources. Of particular note is the Math Images Project, which provides lots of free math related images, organized by topic. Images shared under GNU FDL 1.2, which is just like CC-BY-SA.
“The Math Images Project has begun as a wiki collaboration between the Math Forum at Drexel University and Swarthmore College. The Project aims to introduce the public to mathematics through beautiful and intriguing images available on the web. Using MediaWiki software, the Project encourages a collaborative atmosphere in which contributors find images and write about them both to learn the mathematics involved and to explain it to others. Ideally, the page creators will interact with users of the site to clarify and enrich the material.”