Flickr: Photos from The Library of Congress
The Library of Congress has loaded over 3,000 pictures on Flickr. There are two sets, one of color photos from the 1930's and 40's and one of black and white "news" shots from the 1910's. LoC's collections are wonderful, and having two of them "outside" in Flickr can only help generate interest in the rest of their resources.
These pictures have no known republication restrictions, so they definitely are good for educational use.
Also see LoC's American Memory collections, Prints and Photographs Divisions Images on Popular Topics, and the rest of the Digital Collections.