Always remember to give credit when using images that are not your own. Also remember that giving credit is not the same as having permission. Pay attention to the terms of use for each site. It can vary for each website and even for individual images within a website.

Free Images:

Pixabay
Pixabay contains over 260,000 copyright-free photos and illustrations. This is a great site to find images for your projects.

Wikimedia Commons
Wikimedia Commons contains freely usable media files that can be used by anyone. Make sure to check the licensing information as it may be different for each image on this site.

Academic Sites:

Library of Congress
Search for photos, newspaper articles, maps, etc. This is a great site for primary source images.

New York Public Library Digital Collections
The Digital Gallery provides free access to over 700,000 images digitized from the NYPL's collections, including illuminated manuscripts, historical maps, vintage posters, rare prints, photographs, and more. Some of the images in the Digital Gallery may be subject to copyright.

Smithsonian Images
An amazing collection of photos on history and science. Students and teachers may use these photos for educational purposes. You must credit the Smithsonian and the photographer. See the copyright section on the website for more information on how these photos may be used.