[rcfox]

I'd guess that it's due to signal reflections causing points of constructive and destructive interference.[0]

The shapes aren't very regular, but that could easily be explained by the shape of the room and its contents.

[0] http://en.wikipedia.org/wiki/Interference_%28wave_propagatio...

[rndn]

Is it correct that this is a quantum mechanical effect, i.e. that the wave function of each photon interferes with itself?

[vortico]

Well, yes to both questions, but on a macroscopic scale the wave equation can explain the collective effect of zillions of photons to more accuracy than you'll ever need.