Creative Commons is a nonprofit organization that works "to increase the amount of creativity (cultural, educational, and scientific content) in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing" (http://creativecommons.org/about/what-is-cc).
Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse.
Here is a wiki on best practices for how to give attribution (credit) to images with CC licenses.
Need images for PowerPoints or flyers? Try these sites.