Growing up in the Philippines in the 1990s, Paulo Ordoveza started a website to debunk urban legends that he received in email forwards. “I was that guy who was kind of a stickler for facts,” he says. Fast forward to 2014 and the graphic designer and web developer—working at the time as a contractor for NASA—was attracted by a different platform rife with untruths. He started a Twitter account, @PicPedant, to debunk suspicious photos he would find going viral on the social media site. The self-described “punctilious Internet killjoy” has been at it ever since. Faked, misleading, stolen, or miscredited photos or photo captions exist on most every online platform, with all kinds of subjects. Sometimes they come from accounts deliberately using these methods to gain followers, clicks, revenue, or other aims, but they also get shared by well-intentioned users who don’t know better. You may have them in your own feed, or have shared one yourself. Wildlife and...