My favorite example of a photograph that illustrates a phenomenon is a bubble chamber image.
On the left is the photograph of the event in the Brookhaven bubble chamber which led to the discovery of a charmed baryon. On the right is a diagram of the event.
Reference:
[1] "The Particle Explosion", Oxford University Press, New York, 1987.
[2] http://www.bnl.gov/bnlweb/history/charmed.asp