Now something clearly intentional for sorting like a chapter 1 thumbnail etc. doesn't bother me as much. However, do consider that if you're doing a uniform chapter thumbnail for the sake of making it easier for readers, a reader who wants to go back and find a specific scene for whatever reason is going to have a tougher time trying to remember what exact chapter and page it fell in as opposed to if a thumbnail is a clipped part of a page. I'm personally a fan of going back and revisting certain scenes, so when a feature like thumbnailing which would be ideal for making parts easily found isn't used like such, it makes me a little disappointed.
Besides, I think episode titles with a brief Ch.1 or whatever numerology could serve that purpose better without detracting from the usefulness of thumbnails.
However, what I was generally referring to in terms of repeated images, is when a comic just clearly reuses one image because they're too lazy to make one each episode