YouTube vertical thumbnail size: 9:16 at 2160x3840

YouTube's thumbnail help lists a Shorts thumbnail as 9:16 at 2160x3840 and a video thumbnail as 16:9 at 3840x2160, with a 640-pixel minimum. Files are JPG or PNG. openai/gpt-image-2.5 on Sume can generate exactly 2160x3840. Read 2026-09-29.

What sizes does YouTube list?

The page also says a vertical video with a 16:9 custom thumbnail gets an auto-generated 4:5 thumbnail on home, explore and subscription pages.

How do I generate 2160x3840 with Sume?

openai/gpt-image-2.5 accepts custom image_size with both edges multiples of 16, the long edge at most 3840, ratio at most 3:1 and 655,360 to 8,294,400 pixels. 2160x3840 is 8,294,400 pixels, exactly at the ceiling, and 2160 = 16 x 135, so it is valid. Expect a slow request; large sizes may return a 202 job to poll.

Ask for space: keep text large and centered, since thumbnails shrink.

curl -X POST https://api.sume.com/v1/images \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-image-2.5",
    "prompt": "Bold vertical thumbnail: surprised chef holding a giant croissant, big empty space at top for text",
    "image_size": "2160x3840"
  }'

Can I use a frame from the video instead?

Yes: POST /v1/video-frames returns stills as JPEG at the source size unless you set max_edge (16 to 2160). Note the 2160 cap: a still from a 1080x1920 clip stays 1080x1920, above the 640 minimum but below YouTube's largest listed size.

What are the limits of this answer?

Sume does not upload thumbnails to YouTube. The listed file-size limits are YouTube's and are not checked by Sume.