Below are examples of citations, done in MLA and Chicago style, respectively.
Ernst, Max. L'Ange du Foyer. (1937). Private Collection. Web. Google Image Search. April 1 2010.
Jean Droit. Debout dans la tranchee que l'aurore eclaire, le soldat reve a la victoire et a son foyer (Image). Available from: Illinois Harvest. <http://images.library.uiuc.edu:8081/u?/wwposters,0>. (April 1 2010).
Credit: University of Illinois - Urbana-Champaign Libraries
NoodleTools from the Library homepage will guide you in citing an image based on your chosen style guide. Be sure to use the Library's link to NoodleTools & Style Guides for the most comprehensive version of the software.