Carlos Delgado, Lynx, 2013, via Wikimedia Commons
The expression "a picture is worth a thousand words" can often be applied to political cartoons. These sites contain political cartoons in the public domain.
The following sites are great sources of copyright-free images:
Getty Open Content: Public domain art images from Getty collections.
Library of Congress Prints & Photographs Online: Images from the Library of Congress, now in the public domain.
Wikimedia Commons: Historical and contemporary images contributed by participants. Millions of high-resolution images of art, architecture, design, people, historical events, diagrams, maps, and more.
World Images: Primarily historical images organized into 18 categories, including Women, Science, Cities, Natural World, Science, and more.
NGA Images - National Gallery of Art: Includes more than 20,000 open access digital images, available free of charge for download and use
Archive.org: Extensive website containing free eBooks, images, video, audio, and other media to view and download. Also includes the Internet Wayback Machine, for accessing archived, past versions of websites.
The Commons by Flickr: Images from archives, museums, libraries and research centers such as the New York Public Library, Smithsonian, Library of Congress and Getty Research Institute. The images have no known copyright restrictions.
Photo Graphic Libraries: Public Domain Images: Provides links to public domain and free image collections, film & video available for use in the public domain
Yale University Digital Commons: Yale's digital collections, including images from the Peabody Museum, Center for British Art, University Art Gallery, Library Map Collection, and Walpole Library Prints and Drawings. Item records clearly indicate when an object/image is public domain material and available for open access and use without license fees.