Images taken by an employee of the United States government as part of that person’s official duties are not subject to copyright in the United States and there are no copyright restrictions on the reproduction, derivative works, distribution, performance, or display of the work.
The sources listed below are not the only government sources for images, but also contain some of the largest image collections. Almost all images are in the public domain unless otherwise noted.
A Creative Commons license (also noted as a CC license) allows the copyright holder to retain the copyright to their works while also providing permission for others to use those works under the license conditions. These licenses are not an alternative to copyright. Only the copyright holder can grant a Creative Commons license for the use of a work.
For users, a Creative Commons license means you do not have to seek permission from the copyright holder, as long as you follow the conditions of the license. The user also does not need to perform a fair use analysis, since the permission is covered by the license.
When a Creative Commons license is not available, or the existing license does not permit the use, the user can still perform a fair use analysis to determine if their intended use falls under Fair Use.
Google offers an Image filter to search for art, photos, and other image types on the internet. Under the Tools menu, there is an additional option to limit your search results to specific Usage Rights. Choose between Creative Commons licenses or Commercial & other rights.
Google's Advanced Image Search provides additional fields and limits for locating images on the internet.