When I started searching about using face recognition technology to help find relatives from these images, I stumbled across this site. I wanted to suggest that, if you are not already actively doing it, it could be helpful to ingest these photographs into your system. This sounds like a great way to help about the Library of Congress and at the same time gain some notoriety for your technology.
The usage of photos and face recognition with people search is a very touchy subject as it touches many of the delicates aspects of privacy. We are indeed using face recognition to allow users to better manage the photos within their own family sites. This allows users to enjoy the power of this technology without being afraid of any misuse or privacy abuse.