Citations to images included in image databases (ARTstor, 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 if known, date (database collection if known).

Citations should include the following information (or as much of it as can be easily determined from the source) in addition to the standard citation elements for the print or electronic source of the image of the work:

artist's name, title of the work as given in the source, date, location of the work if known (standard reference or note citation for the source publication).

Basic advice: 

• credit the creator
• provide the title of the work
• provide the URL where the work is hosted
• indicate the type of license it is available under and provide a link to the licence (so others can find out the licence terms)
• keep intact any copyright notice associated with the work

The MLA Style Center includes How do I cite an image found through an online search engine like Google Images?

The Chicago Manual of Style website includes, How do I cite an image?