Oxford Art Online
This database combines all the content from the Grove Dictionary of Art, the Encyclopedia of Aesthetics, the Concise Oxford Dictionary of Art Terms and Oxford Companion to Western Art, to create a core reference database for art-related topics. It is generally the best starting point for background research, with both longer, in-depth, encyclopedia articles and shorter definitions.
ARTstor
Online image library. You can browse by geographic region or art form or do keyword searching. Images are high resolution and may be downloaded or printed for educational use.
Oxford Reference
Another multidisciplinary online library of reference materials, the Art & Architecture category has a broad collection covering classical, renaissance and contemporary art; western, islamic and latino art; aesthetics, modern design, photography, architecture and landscape architecture. Entries are generally brief and consist mostly of short definitions and/or brief essays on topics.