When using images, you must cite just as you do when using quotes or ideas in your research papers. It is not only important but legally necessary to give the creator or owner credit for the image just as you would a writer.

Citing a work of art in Chicago format.
(always check in your syllabus or with your professor for the required course citation style)

• Oxford Art Online
  Includes some images within the entries.

• The Perseus Digital Library
  A digital library of primary and secondary data on Greek and Roman literature, art, and culture.  Use the Artifact Browser to find buildings, sculptures, vases and sites.

• The Beazley Archive
  A research unit of the Faculty of Classics at Oxford.  The free online databae contains notes, photographs, negatives, color prints, and more.

• Early Photography in Greece and the Mediterranean The Getty Research Institute's research guide of 19th- and early-20th-century photographs of ancient Greek and Roman architecture. To search the digitized collection select "View Digitized Works" at the top of the page. Use key words for monuments such as Acropolis or Parthenon, or geographical locations such as Mycenae.

• Watercolour World  Free online database of documentary watercolours painted before 1900.

Also, don't forget that many books, including Lexicon iconographicum mythologiae classicae (LIMC), contain images.  If you cannot find better images online, feel free to scan books using the flatbed scanners on the main floor of the library.