Works that are in the public domain are not protected by copyright because:
Works in the public domain may be used without any limitations.
There are several great resources for finding and accessing materials in the public domain:
Smartcopying - Open Education Resources (OER) and Creative Commons
Images In The Public Domain and Creative Commons Licensed Content
Pixabay - Over 390,000 free photos, vectors and art illustrations in the public domain
Pexels - Over 10,000 free stock photos. All images available under the Creative Commons Zero (CC0) license. Attribution is not required.
Flickr Advanced Search - Choose “Only search within Creative Commons-Licensed Content.”
Google Advanced Image Search - Use the “Usage Rights” field to limit by license type.
Library of Congress: American Memory - A free “digital record of American history and creativity.”
Library of Congress: Prints & Photographs Online Catalog - Photographs, prints, drawings, posters, and
architectural drawings, and more.
NGA Images - Public domain artworks from the collections of the National Gallery of Art.
Noun Project - Free clip art images requiring creator credit.
NYPL Digital Gallery - Illuminated manuscripts, historical maps, vintage posters, rare prints, photographs, and more, from the New York Public Library.
Notable collections: Image Archives, and Smithsonian Images
Wikimedia Commons - Browse or search for freely reusable images.
Wikipedia Public Domain Images - List of public domain image sources on the web.