Looking at a few of your images and the IP addresses, I would say its the Pinterest Bot.
Most of them are in the 54.x.x.x range, and this lists as Amazon (AWS) servers. Pinterest use Amazon servers, and a few of them also show the Pinterest agent string for a few of the IPs on your images, which further confirms Pinterest.
Also, the fact that alot of your images are pinned would add to that theory also.
Every time someone Pins one of your images, you will get a hit from a Pinterest bot, so atleast for your situation, those IPs are probably humans, rather than machine run bots.
Rob.