Artstor is the premier art historical image database, ARTstor is a digital library of approximately 800,000+ images in the areas of art, architecture, the humanities, and social sciences with a set of tools to view, present, and manage images for research. Searching in ARTstor requires no registration, but to access its full suite of tools (uploading and downloading of images, creating images groups, saving images to a private or public library, etc.) registration is required. It's free and painless.
The power of computing technology has brought a flood of images, available on the web with a few clicks. On this page you will find links to a number of digital image collections. Going beyond Google Image searches, these web collections tend to focus on specific types of images, periods, or aspects of visual culture. Like all resources that you include in your papers, presentations, and projects, digital images should be used responsibly. Although it is generally acceptable to reproduce images in your (unpublished) work for class, be sure to comply with the Terms and Conditions of Use provided by the databases or websites that you acquire the images from, and always cite your images, just as you would any other source.
Take a look at the Digital Images Collection Wiki; a database of fair use images from across the web.