Yale Photonegatives Collection
The Beinecke Rare Book and Manuscript Library photo search page. This has large numbers of photos and other images, at fairly high resolution (typically 1400 pixels in the larger dimension). Finding stuff can be a bit hard as the keywording is very spotty. But there are "postcards" to Gertrude "Stein" and Alice "Toklas", photos from the construction of the Union Pacific "Railroad" (many of those are of interesting things along the way), Freud with his "dogs", etc. Every year I tried between 1840 and 1940 had at least one image.
dead link: http://beinecke.library.yale.edu/photonegatives/default.htm