According to [1], they collected half a million photos from instagram, selected 20,000-30,000 photos from each city from that, then filtered the set down by using Mechanical Turk.<p>[1] <a href="http:&#x2F;&#x2F;selfiecity.net&#x2F;#dataset" rel="nofollow">http:&#x2F;&#x2F;selfiecity.net&#x2F;#dataset</a>