YouTube thumbnail limit: 2 MB on mobile, 50 MB on desktop

YouTube's thumbnail page gives two upload caps: 2 MB for a video thumbnail on mobile, and 50 MB for video, Shorts and podcast thumbnails on desktop. So a frame that is fine from a computer can fail from a phone. Export the still as a JPEG and check its size before you upload.

What does YouTube's page say? (read 2026-10-09)

The account must be verified to upload custom thumbnails.

How do you pull a frame from your clip?

Import the clip, then call video_frames with one or more times in at[]. The output is a durable image at source size by default, JPEG unless you ask for PNG, and max_edge can clamp the long edge between 16 and 2160. Billing is by the job's Modal compute, so there is no fixed per-frame price.

Because the clamp tops out at 2160, a 9:16 frame cannot be pulled larger than 2160 on its long edge by this route. That is above YouTube's 640 minimum, so it only matters if you want the 2160 x 3840 size the page recommends; then render the thumbnail as an image at that size instead.

How do you stay under 2 MB?

• Keep JPEG (the default); PNG stills of a busy frame are usually much larger.
• Lower max_edge, for example to 1280 for a 16:9 frame, which is still above the 640 px minimum width.
• Check the exported file's size on disk before uploading.
• If you only upload from desktop, the 50 MB cap makes this moot, so keep the large file.

Which frame should you pick?

Pick a frame with the subject clear and the text large enough to read at small size. To view candidates quickly, extract several times at once, then pick one. The label rules are separate; a thumbnail made with AI is covered in YouTube's label rules.

For a Short, remember the page says custom thumbnails for Shorts are added only in YouTube Studio on a computer, so the mobile cap does not apply there. Keep two exports of a chosen frame, one small JPEG for phone uploads and one large file for desktop, and name them so nobody mixes them up.