More than 20 million tattoo lovers, and the plain curious, flock to Tattoodo’s site each month to peruse its massive collection of ink uploaded by people from around the world. Previously, these uploads had been laboriously hand-categorized and tagged. Then Tattoodo developers Goran Vuksic and Dennis Micky Jensen — tinkering around during some of their company’s regular hacker hours — had the insight to build a neural network for image recognition and apply it to Tattoodo’s collection of body art.