The illustration shows 5 different sets of photos (randomly selected from Flickr) accumulating over successive rows. The first row is a single image. Then 2 images, 5 images, 25 images, and 100 images in the bottom row. I have cranked up the saturation to reveal the orange shift (unprocessed averages tend to look like dirt or milk-chocolate).
I stumbled across this effect in 2006, playing with Flickr, and have blogged about it a few times. Other digital artists who use the same averaging technique have also observed the effect. The reasons why it happens are not yet entirely clear, but I suspect it has something to do with chemistry and physics. Interestingly the same effect occurs with collections of human-generated synthetic, abstract art (not photos), such as fractals.
Over thanksgiving break I wrote an informal paper describing my findings to date, in hopes of attracting some brighter minds to the topic. Perhaps you are one of those minds? You’ll find my paper here.
If you prefer pretty pictures to words, here’s a Flickr set containing some of my image averaging experiments.