I generally trust the long-term stability/accessibility of libraries and archives more than I do something like Flickr. Even if it's not shut down, Flickr could well limit API access or do any number of other things.

IMO the U.S. Library of Congress does it right. They do actually have a Flickr page, for people who prefer that interface: http://www.flickr.com/photos/library_of_congress/sets/

But there is also the canonical digital archive at: http://www.loc.gov/pictures/