Greasy Fork is available in English.
Shows the collection of available thumbnails in the highest possible resolution.
This script shows available thumbnails of YouTube videos and streams under the video. The thumbnail previews are clickable, linking directly to the images.
This script is not intended to be used all the time. Instead, just turn it on in your userscript manager whenever you need access to the thumbnails.
Example screenshots are from videos:
When a thumbnail of a particular resolution/quality isn't available, the default YouTube placeholder is used