Just do a Google search for:
library of congress prints & photographs online catalog
This should bring up the The LOC prints and photographs collection which number more than 13.7 million images. The collection is searchable.
(I'd include a direct link here, but I'm new so I can't yet!)
The photos that are available for download in a high resolution format are generally in the public domain. You'll want to avoid those that you need to order because they generally have copyright issues.
The collection here is amazing! Hope this helps!