Flickr's Commons shares the world's public photography archives. It was launched in January 2008 with a pilot partnership with the Library of Congress. Today, participating institutions include many others, such as those listed below. Links follow for these two major sources are as follows:
A database of over twenty-nine million 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.
Copyright: The legal right, given to an originator or owner of a work. Art and images of works of art are copyrighted. This means that U.S. Copyright Law grants creators of an original work the exclusive right to reproduce and distribute their own work. However, their are certain conditions which permit people the use of a copyrighted work without the creator's permission. See "Fair Use" for more details.
For more information on "Copyright", check out Brown University Library's guide on Images and Copyright by Karen Bouchard at the link below:
Fair Use: The policy that copyrighted material may (under certain circumstances) be quoted or used for purposes such as criticism, news reporting, teaching, and research, without the need for permission from or payment to the copyright holder. This allows scholars, students, teachers, and others to use works that are still in copyright protection for educational purposes. Always verify if you need to obtain permissions prior to assuming that a copyrighted item can be used.
For more information on "Fair Use", go to MIT Libraries' guide for Images and Fair Use by Ellen Duranceau at the link below: