@Pesala Actually, there is an easier way to do this: http://countingcharacters.com/youtube-thumbnail-downloader
The problem is that I have to check EACH video (I have several hundreds of them ＼(º □ º l|l)/) before assigning a thumbnail to it. Earlier, Vivaldi was able to create YouTube thumbnails automatically by pressing 'f5' but now this function no longer exists.
@greibush I assume that you mean the Bookmark Thumbnail as shown in Speed Dial or the Bookmarks Panel, not the icon, so I changed your thread title accordingly.
The icon used will always be the favicon for the web page.
Note that I said "when tabs are already in thumbnails mode" as shown in my screenshot. I such case, it is not necessary to show the popup thumbnails anymore. But when tab thumbnail mode is collapsed again, the popup thumbnails should show again. It should be a smart automatic behavior.