I suppose I'm picky about how my own thumbnails look because I was a graphic design major. So I strive for something that's clear, attractive, and representative of my series (which reminds me, I need to update the thumb for Demon House at some point).
As for chapter thumbnails, I personally don't mind when the same image is reused. For my series though (the one that has chapters), I like the thumbs to be different, because the episode titles have the chapter in them and I like showcasing bits of my art that I thought I did a good job on.