Public Domain resources
(http://en.wikipedia.org/wiki/Public_domain_image_resources)
Wikipedia provides a list of web resources that have, sometimes search-able, databases of public domain images. Many links here are broken, but some seem valuable if they suite your needs. An interesting thing to note is that any work taken by an employee of the Federal Government, while they are employed and at work, belongs to the Federal Government and in turn belongs in the public domain. Read more about it here. Uncle Sam ‘lens’ a hand.
Clandestine will take advantage of section 105 of the Copyright Act, in utilizing images taken by the Federal Government. Including this one: