Many museums have digitized and made their collections available online. Here is a list of some digitized museum collections of interest to this class.
For more details about how to search for images and then how to cite them properly, see the Oviatt Library LibGuide on Finding and Using Images.
The College Art Association publishes a comprehensive guide to understanding Fair Use for the visual arts. You can also consult the Oviatt's research guide on Copyright for more general information.
Bibme.org has an MLA format guide that includes instructions and examples for how to cite photographs. The section for photograph citations is about three-quarters of the way down the page.