Digital media is sometimes challenging to cite -- no matter how up-to-date, your style guide will not have a citation style for every possibility. And having a perfectly formatted citation is not helpful if it does not point back to the creator as close as possible to the source. Feel free to ask a librarian if you ever have questions.
Here are some excellent guides that explain the finer subtleties:
And a comprehensive citation tool: Cite Source (Developed by Trinity College in Hartford, CT, developed through a CTW Mellon Grant for Information Literacy).
Here's the example it gives for citing artwork in the Chicago Style (pdf)
Penfield, E. (1908). Cornell [Poster]. New York Public Library Art and Architecture Collection, Miriam and Ira D. Wallach Division of Art, Prints and Photographs. (Posters. New York, 1890s-1907. / E. Penfield, Leaf 62). Retrieved from New York Public Library Digital Gallery. http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?strucID=341918&imageID=1131286