Auto-generated thumbnail

YouTube samples frames from across the runtime and presents three of them in Studio. Uploading your own image instead requires a phone-verified account. The three stills are picked to represent the footage rather than to sell the click, which is why they so often land on a blink, a half-formed word or a wide shot of nothing much. They are fine as a placeholder while the real image is being made. They are also the usual reason an old back-catalogue video looks unclickable next to everything else on the channel page, and swapping one for a proper thumbnail costs nothing.