"The Commons: Your opportunity to contribute to describing the world's public photo collections."
These collections from institutions and corporations are all published under usage guidelines called "no known copyright restrictions" meaning that they have been unable to locate the copyright owner. These photos are probably public domain. They can certainly be used under fair use guidelines for academic purposes.
The cool thing about this collection of collections is that "the public" are asked to tag the photos to make them more accessible to others.
FYI: At the bottom of the sign in page is this warning:
*Any Flickr member is able to add tags or comment on these collections. If you're a dork about it, shame on you. This is for the good of humanity, dude!!