A quick answer
YouTube recommends 3840 × 2160 at 16:9 and a minimum width of 640 pixels. Its current help lists JPG or PNG, with 50 MB on desktop and 2 MB for mobile video-thumbnail uploads.
16:9 for videos
Video thumbnails use different dimensions from Shorts covers.
Recommendation and minimum
A smaller image may still meet the minimum.
Choose your device
Select the device you will upload from.
How to approach it
Select the image
Use the finished JPG or PNG.
Check dimensions
Read the width and height.
Choose a device
Compare file size with the relevant limit.
Review the export
Inspect text and details after saving.
Understanding smaller resolutions
1280 × 720 also has a 16:9 ratio and exceeds the minimum width. The larger recommendation provides more pixels for larger displays.
Enlarging a blurry source cannot recover missing detail. Inspect lettering after compression.
Separate Shorts and podcast covers
YouTube currently recommends 2160 × 3840 at 9:16 for Shorts. Check cover controls in your YouTube Studio.
A podcast playlist cover differs from an episode's video thumbnail. Video dimensions do not apply to every cover.
Good to know
Must an image exactly match the recommendation?
The recommendation and minimum are separate requirements. Check the file format and ratio too.
Does the tool measure design quality?
It checks technical properties. Judge readability and the message in the preview.