When you are creating digital projects that will be shared beyond your classroom you need to be mindful of using images that you created, that fall within the public domain, or that have been designated with Creative Commons licenses for reuse.
Sources for Images in the Public Domain or with Creative Commons Licenses
Google permits you to search for millions of images from around the world. After you run a basic Google Image search limit your results by selecting search tools and then under usage rights select Labeled for noncommercial reuse.
Complete list of collections from the Library of Congress Prints and Photographs division. Most have been fully digitized. Many are on topics relating to American history - wars, race, immigration, slavery, and more. Also includes materials with an international scope.
The following is a very small list of sites that provide a large number of public domain and Creative Commons Reuse images.