Fair Use Images
Whenever possible, you should use pictures that are fair use and in the the public domain; however, mistakes are made. To be on the safe side, be sure to include this statement after your works cited information: This presentation may contain copyrighted material used under the educational fair use exemption to U.S. copyright law.
CC Search: Searching through this site should help you access fair use pictures.
Flick: Creative Commons: Flickr has a Creative Common option when searching for images. Since you're creating not-for-profit, educational material, you can stick with the most general option (Attribution).
Wikimedia Commons: Wikimedia Commons is the media section of Wikipedia. All material on Wikipedia is CC-licensed with attribution (you must cite but you can't alter it or make money).