The Digital Public Library of America (DPLA) brings together the riches of America's libraries, archives, and museums, and makes them freely available to the world. Individuals can access a trove of tens of thousands of images across these institutions through DPLA's search interface (by selecting the "images" filter on results after performing a keyword search). Alternatively, consider using the OpenPics app. (http://dp.la/apps/6) to find images from the DPLA (and other repositories).

http://dp.la/