I was hoping to also see a covariance matrix, or maybe even a 3d histogram of pixel colors on the web -- that would be interesting. This person is clearly not a data scientist :)
With random images which include a few outdoor photos, there is a noticeable effect from the sky, but not as dramatic as I might expect - it tends to produce a whitening, rather than a visible blue.
If I'd been asked, I'd perhaps have tried to be clever and said that the top half of the image would be blue as the sky is blue and indoor walls are usually white which won't hurt the blue so much, and that the bottom half would be a browny green due to grass and carpet etc.
Obviously, this is just another explanation.
Actually I would of liked to of seen some of those aggregate photos. Maybe there WERE graduations!
edit: wish granted!: http://jbum.com/papers/EmergentOrange_paper.pdf
some of his sample sets did produce graduations, in particular vignettes and lighter at the top darker at the bottom themes.
in defence of my statement is figure 10
Figure 10. Averages of four different pools of digital
abstract art found on Flickr. Pools used: Processing,
CGArt, Computer Art Creations, Generative and
Evolutioanry Art
4) It’s caused by chemistry & physics. The amalgam photos
are functioning as a kind of mass spectrometer,
reflecting the average chemical composition of the
subjects being photographed.
[...]
I find #4 the most convincing. It is not necessarily
mutually exclusive with some of the others (e.g. the
chemistry of the earth/sun affects human choices and
camera design).