Stock photo links resource!  Site quickly explains difference between  Creative Commons License and Public Domain photos.  Photo stock websites are given short description and rated on a four star basis.  Sites most useful for history include: New Old Stock, Smithsonian, Photo Everywhere, Life of Pix, Little Visuals, Magdelenie, and Splashbase. 

The usual suspects, plus a few more.

I'm always in the market for free images - this looks great!