A pretty thorough estimate of the number of unique images (photos and illustrations) on the entire internet, broken down by source.
Most of the larger estimates are accurate to within a billion or so, the smaller ones to within about 25%, where figures are exact they've been retrieved somewhere between Oct 2011 and April 2012.

Loading more stuff…

Hmm…it looks like things are taking a while to load. Try again?

Loading videos…