YouTube Shorts custom thumbnail: Studio on a computer, 9:16 still

For a Short, YouTube asks for a 9:16 thumbnail of up to 2160 by 3840 pixels, with a minimum of 640 pixels on the short side. Adding a custom thumbnail to a Short is currently only available in YouTube Studio on a computer, so an upload script from a phone workflow will not set it.

What are the Shorts rules?

The thumbnails page and the Shorts upload page together give these limits.

How do I export a vertical still?

Extract a frame from the vertical master. video-frames keeps the source size unless you clamp it, so a 1080 by 1920 master gives a 1080 by 1920 frame, which clears the 640 minimum.

curl -X POST https://api.sume.com/v1/video-frames \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: short-thumb-001" \
  -d '{"video_url": "https://media.sume.com/artifacts/artf_demo/talk.mp4", "at": [1.5], "format": "png"}'

What is the catch?

The Sume frame cannot exceed a 2160 long edge when you clamp it, and the YouTube maximum is 3840, so a Sume frame is smaller than the largest accepted size. That is allowed: the minimum is 640. The thumbnail may also be replaced by an auto-generated one on some pages.