Citations can be formatted according to the citation style you are using (MLA, APA, Chicago Manual of Style). Using images that you did not create require citation in all cases and the citation should be accessible in the context of the image's use.
Here are a few basic guidelines:
creator's name
title of the work as given in the database
location of the work (museum, library or owning institution if known)
date
(database collection [if known])
rights information, if known