A Guide to the Web's Growing Set of Free Image Collections

Europeana badly needs a mention: http://www.europeana.eu/ , https://en.wikipedia.org/wiki/Europeana

More than 2000 european institutions contribute to the growing collection.

Also has an API: http://www.europeana.eu/portal/api-introduction.html

The Rijksmuseum even has an API: https://www.rijksmuseum.nl/en/api/instructions-for-use