I recently came across the excellent website
http://publicdomainreview.org/ which curates public domain resources and holds large collections of non-copyrighted images.
Through the website I learned about the British Library's vast public domain collection, uploaded to Flickr Commons under the public domain mark, meaning complete freedom of re-use. To learn more, here's the website post: http://publicdomainreview.org/collections/the-british-librarys-mechanical-curator-million/ and the library's incredible "Mechanical Curator" http://mechanicalcurator.tumblr.com/