Where We Take Our Pictures
The heatmaps are based solely on the number of available Panoramio photos for an area: both the number of photos and the number of authors is taken into account.
The dark and the blue areas have a few photos, the red areas have more and the yellow areas have a large number of photos geotagged.
Images: Screenshots from Sightsmap, the first shows a heatmap of most popular photographed areas. The second overlays photos pulled from Panoramio on top of the Google map.
H/T: Visualizing Data.