First frame

In the Shorts feed there is no thumbnail before playback. The video simply begins, so whatever is on screen in that first moment is the whole of your packaging. Thumbnails on Shorts still matter for the channel page and for search rows, though most Shorts views arrive through the swipe feed where they are never seen at all.

Treat the first frame as a design decision. A clear subject, legible text if the idea needs words, and no logo animation or fade from black eating the seconds that decide whether the viewer stays.