Here are some ways to search for Creative Commons-licensed images: Google images search, use the Search Tools -> Usage Rights option,
http://Flickr.com using the License filter option. Another is
http://search.creativecommons.org/. For more options, visit this page on the library website:
http://www.mchslibrary.org/images-and-sounds.html. Double-check that the image you found has a Creative Commons license. It will say: "Some rights reserved." "All rights reserved" images are copyrighted and NOT okay to use in published works without permission from the copyright holder.