Locals and Tourists.

Blue points on the map are pictures taken by locals (people who have taken pictures in this city dated over a range of a month or more). Red points are pictures taken by tourists (people who seem to be a local of a different city and who took pictures in this city for less than a month). Yellow points are pictures where it can't be determined whether or not the photographer was a tourist (because they haven't taken pictures anywhere for over a month). They are probably tourists but might just not post many pictures at all.