Google Image Search: Advanced search feature allows you to narrow results by usage rights. Choose the appropriate setting (most common academically is the “free to share or use” option. If you are going to make changes or adapt the photo choose “ free to share or modify” instead. You can allow filter the search by size, color, or type such as photo, clip art, or animated drawing.
Wikimedia Commons: Part of the Wikipedia family, Wikimedia contains images, video and audio files that are explicitly free to use with public domain and freely-licensed educational media content. Click on the image to view copyright parameters.
Flickr: Flickr does not search the web for images but is an online photo management site with individual and institutional participants. Be sure to change the license settings from Any License to Creative Commons Only. Hosting various institutions “no known copyright restrictions”, The Commons on Flickr is also useful for historical images.
