The ARTstor Digital Library provides more than one million images with tools for teaching and research, and now includes images from the Vision database from the Visual Resource Centre. Create an account using your Western email address in order to create folders of images, and to download high resolution images.
Google Art Project
Google has partnered with museums from around the world to allow users to explore museums using Google's "Street view technology". Extreme details can be viewed for major works of art.
The Metropolitan Museum of Art Collection
Access to over 400,000 digital images. Some of the images are in the public domain and are available for download and reuse in non-commercial publications. Look for the OASC tag for images that are available for reuse.