If the image is only found on the web, provide the name of the artist, title of the work then follow the citation format for a website. (Remember to delete "http://" from the address, it is not needed and makes the citation easier to read.) If there is no artist information leave it out.
If the the picture was found using Google Images do NOT cite Google Images as the publisher, visit the original page and use information from the website that is hosting the picture.
Last Name, First Name. "Title of the Image." Title of Website, Version (if applicable), Publisher, Publication Date, URL.
Souza, Pete. "President Obama, Vice President Biden and Members of His National Security Team." PHOTOS: One Photographer's 'Remarkable' Chance To Cover The President, NPR, 26 Oct. 2016, www.npr.org/2016/10/26/499490029/photos-one-photographers-remarkable-chance-to-cover-the-president.
If the work is posted under a username, use the username for the author.
Enlightened Fellow. "Neither Here Nor There." Flickr, 6 Oct. 2012, www.flickr.com/photos/enlightenedfellow/11942597383/in/pool-best100only/.
If you are using a digital image of artwork that is also found in a museum, include the institution and the city where the work is housed.
van Gogh, Vincent. Self-Portrait with a Straw Hat (obverse: The Potato Peeler). Metropolitan Museum of Art, New York. The Met, www.metmuseum.org/art/collection/search/436532. Accessed 3 Apr. 2017.
When citing the original artwork (like a painting you saw in a museum):
van Gogh, Vincent. Self-Portrait with a Straw Hat (obverse: The Potato Peeler). 1887, oil on canvas, Metropolitan Museum of Art, New York City.
Here is an example of how to cite the same painting but from a picture in a book. (Note that the museum description and book description differ slightly):
van Gogh, Vincent. Self-Portrait, Metropolitan Museum of Art, New York City. Van Gogh in Arles, by Ronald Pickvance, Harry N. Abrams, p. 34.