Below are places online to find copyright free or "fair use" internet images.
Will search various internet engines (including Google, Wikipedia, Flickr, Fotopedia, Open Clip Art Gallery) for images you are free to reuse.
Hosts only freely licensed images, sound files, and other media. With over 11 million uploaded files, it is the largest repository of free images and it contains many public domain images.
Wikipedia: Public domain image resources
This site will provide you with links to many sources of public domain images on the internet, including some of the ones below.
Google Image Search
After you do a Google image search, click on "Search tools" and then "Usage Rights" and then "labeled for reuse." You will have access to images that are free to reuse and not subject to copyright restrictions. Numerous websites offer copyright-free and public domain images. Most require only that attribution be given to the originator (i.e. ALWAYS cite your source
!). Check each site for conditions of use. Materials (maps, photographs, diagrams, digital images, reports, books, etc.) created by employees of the federal government while on the job is in the public domain. Regardless of the site, always check for copyright and use restrictions. Some of the materials do have some restrictions and you need to be aware of them. For example, some of the Smithsonian images must be used as is with no modifications.