Image Source: "Henry Ford Bridge, Spanning Cerritos Channel, Los Angeles-Long Beach Harbor, Los Angeles, Los Angeles County, CA." by Historic American Engineering Record, Creator, Joseph Baermann Strauss, and Ross Construction Company in the public domain.
The public domain includes works that are not protected by copyright, and therefore available to the public. Works can pass into the public domain for several reasons:
Visit the Public Domain Sherpa for detailed information on public domain topics, including questions about images and other media.
What is in the public domain can be a complex question because the laws have changed over time, and there are no comprehensive lists of what has passed inot the public domain.
Luckily, there are several tools on the web to help you figure it out.
Library of Congress Photostream - historical images from American history and culture
Project Gutenberg - a collection of copyright free downloadable books
Science.gov - a searchable database of scientific information produced by U.S. government agencies.
Image Source: "Portrait of Billie Holiday, Downbeat(?), New York, N.Y., ca. June 1946" by William Gottlieb in the public domain.