APA format
Model:
Creator's last name, first initial. (Role of creator). (Year of creation). Title of image or description of image. [Type of work]. Retrieved from URL/database
Examples:
Salgado, J.F. (Photographer). (2010). Up close and personal with the very large telescope. [Digital image]. Retrieved from http://www.eso.org/public/images/potw1049a
Sayer, L. (Photographer). (2007). Nurse log. [Digital image]. Retrieved from http://www.flickr.com/photos/riss/368673880