Format: 

Artist, A.A. (Year). Title of work [Medium]. Retrieved from URL

Artist, A. A. (Exhibit Date). Title of work [Medium]. Title of Exhibit. Repository, Location. (Original work Date)

Examples: 

Turner, J. M. W. (1845). Norham castle, sunrise [Painting]. Retrieved from http://www.tate.org.uk/art/artworks/turner-norham-castle-sunrise-n01981

In-text: (Turner, 1845)

[Photograph of Sagrada Familia]. (n.d.). Retrieved from http://www.tripandrate.com/en/barcelona/places/gaudi/sagrada-familia

In-text: (Photograph of Sagrada Familia, n.d.)

Eder, J. M., & Valenta, E. (2015). Zwei goldfische und ein seefisch (Christiceps argentatus) [Photograph]. A History of Photography. George Eastman House, Rochester, NY. (Original work ca. 1896)

In-text: (Eder & Valenta, 2015)

Notes: 

If a title is not available, provide a brief description enclosed in square brackets. Similarly, if the date is not known, but you can provide a "best guess;" enclose the date in square brackets and precede your estimate with the abbreviation for circa (ca.) - [ca. 1937]. See the second and third examples for more information.

Source:
APA (ER) - no. 51, 52, 73