Artstor is a digital image library that provides over 1.6 million images in the arts, architecture, humanities, and sciences from around the globe and across time periods. Included are images from international museums, photographers, libraries, scholars, photo archives, and artists and artists' estates. Register for free with the site to access its full range of services.
Associated Press Collections Online is a publishing program focusing on making varied treasures of the Associated Press Corporate Archives, AP Images, and AP Archive available to libraries worldwide.
A database of 29,460,574 freely usable media files to which anyone can contribute. These are not necessarily art images, but it can be a good source for buildings, outdoor monuments, and architecture.