The top figure is for the northern hemisphere, with the summer solstice in June. The bottom is for the southern hemisphere, with the summer solstice in December. The south has a much less clear pattern, but perhaps because it only has 8% as many pictures represented.
July 4 is the day when people are especially eager to take pictures after dark, and Halloween the somewhat less prominent one (but going until later). People also take pictures somewhat late on Christmas Eve and early on Christmas Day. New Year's Eve/Day is hard to see because it is the edge. There is a dark stripe for February 29, which only happened in one of the three years plotted.
The noise at midnight is from pictures that are tagged with a date but not a time of day. The vertical stripes are from the weekly pattern where people take many more pictures on weekends than on weekdays. The brightest points are 8000 photos per 15 minutes in the north, 645 in the south. Times are photographers' local time.
Data from Flickr, January, 2008 through December, 2010.