Europeana Photography just unveiled a 2.3 million artifact collection that “includes images and documents from 50 European institutions in 34 different countries.” It seems that most are either in the public domain or available for use under a Creative Commons license. They have it set-up so it’s extremely easy to see what, if any, copyright restrictions apply to each photo.
You can read more about it at Open Culture.
This collection is part of something called Europeana Collections, which has similar sites on art, fashion and music.
I’m adding this info to The Best Online Sources For Images, but not to The “All-Time” Best Sources Of Online Images.