16:9 is the ratio of nearly every video surface: a 1080p video frame, a slide, a widescreen monitor, a YouTube player, an OG image, a Twitter/X card. The number people usually want is 1920x1080 (divide by 9, multiply by 16 — 1080 is the height, 1920 the width), but that is not the only correct answer and it is often not the right one. 1280x720 is the same ratio at two thirds the pixels and roughly half the file weight, which is a better fit for a web hero or an email header where the display width never exceeds a few hundred CSS pixels. YouTube publishes its own thumbnail target and it is larger than a video frame: 3840x2160 for videos, with a 640 px minimum width, and a hard ceiling that depends on where you upload from — 2 MB on mobile, 50 MB on desktop. Note what that means: the thumbnail is meant to be stored at four times the size of a 1080p frame, because it also serves as the preview image in the embedded player. Resize to the ratio at the size the destination uses, then compress to a byte target, because a 3840x2160 PNG export can be several megabytes on its own.