The maps are ordered by the number of pictures taken in the central cluster of each one. This is a little unfair to aggressively polycentric cities like Tokyo and Los Angeles, which probably get lower placement than they really deserve because there are gaps where no one took any pictures. The central cluster of each map is not necessarily in the center of each image, because the image bounds are chosen to include as many geotagged locations as possible near the central cluster. All the maps are to the same scale, chosen to be just large enough for the central New York cluster to fit.
Additionally, trace color indicates mode of transportation. Black is walking, red is bicycling, and blue is moving by motor vehicle. From what I gather, photos either come straight from Flickr or a teamed group of people. Unfortunately, that's all I can find though. Some more explanation would probably make these a lot more enjoyable. Nevertheless, they're nice to look at.