Ha, as a deeply unartistic person (can't draw to save my life), I've just gone with the same picture of the Blue Ridge Mountains for my chapter thumbnails, except they grow very gradually darker (hint hint at things to come). I did make something a bit fancier for the book thumbnail with Canva, though.
As a reader, I don't think changing your thumbnail is especially necessary, even if you're doing a comic rather than a novel. I like that little spark of recognition I get when I spot a familiar thumbnail. I like it when they change, too, as it's always fun to see what the new design looks like, but I definitely don't think keeping the same thumbnail for a longer period of time is something people should worry about.
