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Saw that Watame changed her thumbnails this week.
My song at least 3 times and Shishi Wata Cooking twice, but it seems to have settled to this.
Maybe it's an artistic choice, but all the thumbnails have been just frames from the videos with the title added.
Doesn't look like it had much of an impact but, changing thumbnails to something more eye-catching is a good wat to boost exposure numbers.
Veritasium even does it to 2-3 year old videos every now and then and they get a new wind in the YT algo.