From a PBS special, this website highlights images of American photography. Companion book available here. Specific sections on Social Change, Cultural Identity, Digital Truth, and Persuasian provide good information. Also look at the Image Lab for technical tricks used to manipulate images.
Contains catalog records and digital images representing a rich cross-section of still pictures held by the Prints & Photographs Division and other units of the Library. Includes more than 50% of the Division's holdings, about one million digital images
NYPL Digital Gallery provides access to over 520,000 images digitized from primary sources and printed rarities in the collections of The New York Public Library, including manuscripts, historical maps, vintage posters, rare prints and photos, and more
The California State University IMAGE project contains over 50,000 images, is global in coverage and includes all areas of visual imagery. WorldImages is accessible anywhere and its images may be freely used for non-profit educational purposes.