MLA style guide provides the following reference for electronic images:
Provide the artist's name, the work of art italicized, the date of creation, the institution and city where the work is housed. Follow this initial entry with the name of the Website in italics, the medium of publication, and the date of access.
Goya, Francisco. The Family of Charles IV. 1800. Museo Nacional del Prado, Madrid. Museo National del Prado. Web. 22 May 2006.
Klee, Paul. Twittering Machine. 1922. Museum of Modern Art, New York.The Artchive. Web. 22 May 2006.
If the work is cited on the web only, then provide the name of the artist, the title of the work, the medium of the work, and then follow the citation format for a website. If the work is posted via a username, use that username for the author.
brandychloe. "Great Horned Owl Family." Photograph. Webshots. American Greetings, 22 May 2006. Web. 5 Nov. 2009.