In addition to the resources listed in the Art History guide, you may want to search the database
FADIS. This site requires a login and password that was supplied to your professor. Contents are primarily visual images with some accompanying text supplied by participating members for pedagogical purposes.
Tate Galleries (includes Tate Modern, Tate Britain, Tate Liverpool, Tate St. Ives). Some images are available without a subscription. The images are released under a Creative Commons license--follow the guidelines provided.
The Internet Archive. The archive is perhaps most useful for finding texts (digitized books and pamphlets) that are in the public domain. Remember that texts often include images: frontispieces, photographs, portraits, illustrations, book covers, etcetera. Images, audio files and other formats are also archived here.
Google and other search engines remain hugely important source of images. Consider using Google Advanced Image Search that allows filtering by Usage rights: free to use or share.
Wikimedia Commons may contain images that are in the public domain or for which the creators have given permission for non-profit use.
Flickr, specifically images with Create Commons attributions.