YouTube will allow you to filter your results by Creative Commons licenses.
Google will allow you to limit your Image Search results by license. Click on Search Tools for more options.
Wikimedia Commons is a database of freely usable media content, including images, to which anyone can contribute.
Smarthistory collects essays and videos about art history and culture, created by Art Historians and other specialists and licensed 
Most of the images in the Library of Congress Prints & Photographs Collection are in the public domain. However, it is the users responsibility to determine if there are any applicable copyright restrictions. Learn more information here.
Flickr is a photo-sharing site. Go to http://www.flickr.com/creativecommons to search or browse images within each Creative Commons license type.
With Pixabay find, share and use photos, clipart, and vector graphics.
Works on Pixabay have been released into the public domain. You can use any image from this website in digital and printed format, for personal and commercial use, without an attribution requirement to the original author.