http://natgeofound.tumblr.com/It's an archive of selected national geographic photos. It's all over the place, but runs throughout the magazine's history, so it's useful for everything back about the colonial era.
If you want to search for a particular era - say WW2, or the Terra Nova expedition, or Peru - everything is tagged.
Me, I just scrolled through the whole thing. An hour and a half very well-spent.