A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone.
More information available at:
Works fall into the public domain for three main reasons:
1. the term of copyright for the work has expired;
2. the author failed to satisfy statutory formalities to perfect the copyright or
3. the work is a work of the U.S. Government.
As a general rule, most works enter the public domain because of old age. This includes any work published in the United States before 1923. Another large block of works are in the public domain because they were published before 1964 and copyright was not renewed. (Renewal was a requirement for works published before 1978.) A smaller group of works fell into the public domain because they were published without copyright notice (copyright notice was necessary for works published in the United States before March 1, 1989).
Use the Copyright Slider Tool to determine is a work is still protected by copyright.
Many public domain resources can be found on the web. For more infromation on images. Such sites include:
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.
NYPL Digital Gallery - Illuminated manuscripts, historical maps, vintage posters, rare prints, photographs, and more, from the New York Public Library.
US Government Photos & Images Public domain images by topic.
Notable collections: NLM History of Medicine, US Fish & Wildlife Service National Digital Library, National Park Service Digital Image Archives, and Smithsonian Images.
Wellcome Images - All images are made available under Creative Commons licenses.
Wikimedia Commons - Browse or search for freely reusable images.
Wikipedia Public Domain Images - List of public domain image sources on the web.