Read This First
Most images are owned by someone. You are responsible for fair use of any images you download or post. Before you use any images you find on the Internet, read the copyright notice. Confused about copyright, fair use, permissions, and licensing? -- see the library's guide to copyright.
Search Engines for Images
Image and Sound Databases from UC Berkeley
Selected Image Collections
American Memory from the Library of Congress
California Heritage Collection from UC Berkeley
California Views historical photos from local commercial photographer
Life Magazine Photo Archive hosted by Google
NASA Images from the National Aeronautics and Space Administration
Prints & Photographs Online Catalog from the Library of Congress
WorldImages from California State University
You can also try a Google search on "public domain images" to find websites that offer free images you can use in your work.