Beazley Archive at Oxford University
The Archive contains the world's largest collection of photographs of ancient Greek painted pottery, as well as relevant books and offprints, extensive material on the history of gem-collecting, and thousands of other documents and photographs relating to classical archaeology and to Sir John Beazley. Many of the Archive's images are available online, but the Archive's library remains a resource of central importance for research on classical art.
The Portable Antiquities Scheme is a project to encourage the voluntary recording of archaeological objects found by members of the public in England and Wales. Every year many thousands of objects are discovered, many of these by metal-detector users, but also by people whilst out walking, gardening or going about their daily work.
Ancient World Image Bank through Flickr is a collaborative effort to distribute and encourage the sharing of free digital imagery for the study of the ancient world.
ARTstor contains nearly 1.4 million images in the areas of art, architecture, the humanities, and social sciences. Most (but not all!) of ARTstor's images are of archival or museum materials. You must register on campus to be able to download images.
More options for finding images: Guide to Finding and Using Images Online
This guide to not only has excellent information about citing images but also will give you an ArtStor model citation:
Also useful is Cornell's Guide to Citing Images.