Royalty-free images are professional and hobbyist photographs, drawings, and paintings that are available for free use. Normally, use of these images is governed by a Creative Commons Licenses, which is composed of multiple elements. Other images are without copyright (in the public domain) and include images created by the Government of the United States and materials produced or published before certain dates. More information about United States' Copyright rules can be found on the Stanford University Libraries' very thorough guide to Copyright & Fair Use.
Vector graphics allow you to build works of art and functional graphics that can expand to any size. While these collections are focused on vector graphics, you can search some of the other image websites on this page for vector-based graphics.
Creative Commons is a standardized licensing tool that offers creators the opportunity to license their in-copyright works for free use by third parties. Creative Commons offers six copyright licenses. These licenses are:
They also provide tools that work in the “all rights granted” space of the public domain. TheirCC0 tool allows licensors to waive all rights and place a work in the public domain, and their Public Domain Mark allows any web user to “mark” a work as being in the public domain.
Curated collections of Open Access photography.
Not all content on Flickr is available royalty-free or under Creative Commons licensing. These landing pages offer users ways to access that Creative Commons' content easily.