The basic format is as follows:
Author, A. (date). Title of document [Format description]. Retrieved from http://URL
For example, if you wanted to use this image:
You would cite it as follows:
UT Arlington Library. (2006). LibraryFountain-1 [Photograph]. Retrieved from http://www.flickr.com/photos/utarlingtonlibrary/6807530044/in/photostream/
Here is an APA Style Blog entry on citing something you found on a website:
http://blog.apastyle.org/apastyle/2010/11/how-to-cite-something-you-found-on-a-website-in-apa-style.html
And specifically, here is an APA Style Blog entry how to cite artwork:
http://blog.apastyle.org/apastyle/2010/04/theres-an-art-to-it.html
If you are using a website image that is not considered fine art, use either “[Photograph]” or “[Image]” or another descriptive term after the title of the image. If the image has no official title, you can make up a description and enclose it in square brackets to use in place of the title.