A recent enquiry about sources for historical photographs of London which could legitimatelybe re-used for a non-commercial project led me to look at the Europeana website. For anyone not familiar with Europeana already, this is an ambitious EU project to digitise European culture and make it available. The searchable database now contains many re-usable images, often covered by a Creative Commons Licence.
UCL has also contributed material, notably through the Europeana Travel Project, in the form of historical images from the Library of the UCL School of Slavonic and East European Studies (SSEES).
Another excellent website when searching for re-usable images is the University of Nottingham Xpert database.