Thomas, Ruth S. Finding Images on the Web. Boston University Libraries. Last updated 2012.
A Review of Image Search Engines. JISC Digital Media. February 2008. Evaluation of major search engines with image search options as well as specilized image search engines. Includes links to many image sites.
OAIster Academically oriented digital resources harvested from educational and research institutions, national libraries, and other organizations. Select "Narrow your search by format": visual material."
Europeana Provides access to millions research-oriented images, texts, and audio files in libraries, museums, archives, and other research institutions all over Europe.