Students and teachers alike often go first to Google images for pictures, but not all images are copyright free. Why not go directly to a few good public domain sites? Works in the public domain may be used freely and are not protected by copyright. Here are a few such sites. Many states have digital collections of their history.
American Memory Collection - A digital record of American History from the Library of Congress. Find spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American experience. http://memory.loc.gov/ammem/index.html
Calisphere - A World of Primary Sources and more from the University of California. Designed for K-12 students and teachers. http://www.calisphere.universityofcalifornia.edu/
University of Colorado Digital Library – Digital resources for educational use. https://www.cu.edu/digitallibrary/
U.S. Government Photos and Images – A mix of public domain (copyright free) and licensed images. Read disclaimers on each site. http://www.usa.gov/Topics/Graphics.shtml
Flickr creative commons – Some Flickr contributors let you use their image if you give them credit. See various creative commons licensing agreements. http://www.flickr.com/creativecommons/