The poster frame is the still image people see before they press play: on the tile, in their library and on Share Links.
Change it
Click the ⋮ menu on the video's tile and choose Settings, then Poster Frame.
To use a frame from the video, click Choose a Frame, play or scrub to the moment you want, and click Use Current Frame.
To use your own image, upload it instead.
You can change the poster frame on any video, including long ones.
HDR video
Frames taken from HDR video get the same colour correction as the video itself, so the still matches what plays.