The Library of Congress has put two collections of digital images on Flickr: 1,600 images from the Farm Security Administration/Office of War Information and around 1,500 images from the George Grantham Bain News Service. The images can be found at The Library of Congress' Photos.
Regarding copyright, LC says:
Although the Library of Congress does not grant or deny permission to use photos, the Library knows of no copyright restrictions on the publication, distribution, or re-use of these photos. Privacy rights may apply.
See the FAQ for more details.