Google Images is a powerful and expansive image search engine. Use the advanced search option to limit by content type (line drawings, photographs, faces, etc); file size for higher quality images; and usage type (creative commons, etc).
Googe Image Swirl is a different way of displaying your search results. It "organizes image search results based on their visual and semantic similarities" and "combines a variety of image similarity features with additional metadata about the images to build a hierarchy of clusters of image search results." Here's an example of what this looks like using the query "Warhol." Explore this tool to enhance your image searching!
The Getty Provenance Index allows you to conduct provenance research for western European works of art from the late sixteenth to the early twentieth. Originally designed as research tool for the ownership history of individual masterpieces, the index also allows scholars to model complex market developments, social networks, and cultural transfers.
2,300 links to selected arts & humanities ejournals. Journals listed are either free, or offer significant free content.