The YouTube thumbnail size
A custom YouTube thumbnail should be 1280 x 720 pixels (16:9), at least 640 pixels wide, under 2 MB, in JPG, GIF or PNG. It is the main thing people see before they decide to watch a launch or demo video.
How to make a good YouTube thumbnail
Use a close, readable subject and three or four words at most. Check it at the small size it appears in search and suggested videos, and keep text clear of the bottom right corner, where the video length is shown.
Questions
What size is a YouTube thumbnail?
A custom YouTube thumbnail should be 1280 x 720 pixels (16:9), at least 640 pixels wide, under 2 MB, in JPG, GIF or PNG.
What size should a YouTube thumbnail be?
1280 x 720 pixels, 16:9, under 2 MB. It must be at least 640 pixels wide.
Is my image uploaded anywhere?
No. The image is read and resized in your browser; nothing is sent to a server.
How does the resize work?
It scales the image to cover the target size and crops the overflow evenly from the edges, so nothing is stretched.