Europeana 

30 Million items from a range of Europe's galleries, archives, libraries and museums.

World Digital Library

The World Digital Library (WDL) is a project of the U.S. Library of Congress, carried out with the support of the United Nations Educational, Cultural and Scientific Organization (UNESCO), and in cooperation with libraries, archives, museums, educational institutions, and international organizations from around the world.

AP Images

Over six million images from the Associated Press, one of the largest and oldest news organizations in the world.  Images date back to the mid-nineteenth century and more than 3,500 new photographs are added daily.

Artstor

The Artstor Digital Library is a nonprofit resource that provides over 1.8 million digital images in the arts, architecture, humanities, and sciences with an accessible suite of software tools for teaching and research.

Google Image Search

Google provides both basic and advanced image searching for over 1 billion images. Google's Advanced Image Search searching allows you to limit results based on usage rights, by size and quite a bit more.

Digital Public Library of America (DPLA)

Provides for searching across a massive number of digital collections located in libraries, archives and museums throughout the United States.  Includes georeferences and timelines.

Flickr Commons

Flickr provides a search specifically designed to locate works that are offered under the Creative Commons license.

Search.creativecommons.org offers convenient access to search services provided by other independent organizations. Note that selecting different search options within the result list—particularly Image search for Google and Yahoo—may lead to the inclusion of results which are not Creative Commons licensed.

American Memory - Library of Congress

Contains over 5 million images and other archived materials relating to American culture and history. Guidelines for citing and reproducing images are also included.

Wikimedia Commons

Associated with the Wikipedia, Wikimedia Commons searches the images available in the online Wikipedia encyclopedia. Click on an image to view copyright restrictions.