Citations to images included in image databases (ARTstor, Insight, AP Images, etc.), should include the following (or as much of it as can be easily determined from the source):
creator's name, title of the work as given in the database, location of the work, date (database collection [if known]).
Example:
Daniel Chester French, Angel Death Staying Hand Sculptor, Forest Hills Cemetary, Boston, 1893 (ARTstor, The Image Gallery).