[mikub]

What works for me most of the time on youtube is to just avoid clicking on videos where people put their faces on the video thumbnail, those things always make me angry when I see them. Fortunatly youtube stoped showing me suggestions on the frontpage because I disabled watch history, so I only see the thumbnails on the sidebar when watching a video.

[reportgunner]

You can just block all images on youtube using ublock origin and then you won't see thumbnails at all.