Google Images - There is a subset of pictures on Google Images which are under Creative Commons. To find them, first use a search term within images.google.com then narrow your search by clicking:
Tools -- Usage Rights -- Creative Commons Licenses
Creative Commons search
Flickr - To find the copyright free images, use a search term to find photos on your subject, then use the drop down menu under “Any License” to choose “creative commons” or “modifications allowed”
PhotosForClass.com (free for educators and students)
Library of Congress Free-To-Use Sets
New York Public Library Digital Collections - must look under the image to determine the usage restrictions
Metropolitan Museum of Art Open Access Collection
PublicDomainPictures
- choose the free download option under the photo