Guidelines for Fair Use of Copyrighted Materials:
University of Chicago Libraries: Fair Use and Other Educational Uses
Education World: The Educators Guide to Copyright and Fair Use
Creative Commons Resources:
Creative Commons Search: Returns search results labeled for use and resuse
Or, try these resources:
Stock Photography
Unsplash : Use photos listed under the Creative Commons heading
Flickr : Various licenses for different photos
Morguefile: Use photos listed under "Free Photos" tab
Primary Sources
Digital Public Library of America: Photos, letters, paintings, and more
New York Public Library Digital Collections:Photos, letters, books, paintings, music, and more
Library of Congress for Education: Primary source sets, lesson plans, and more
Clip Art
Open Clip Art Library: Download free and free-to-use clip art