Photography, as Susan Sontag famously pointed out, has long been seen as a form of evidence. That spirit seems to animate the trend — spreading and evolving since summer 2017 — of posting “Instagram vs. real life” diptychs that demonstrate how photos, side by side, can become evidence against themselves. Popular in the wellness community, these show food or physiques, spectacularly camera-ready on one side and quotidian on the other. One may depict a brightly colored, Instagram-ready smoothie...