I do the same as you. I just use the a smaller version of the chapter cover for episode thumb nails.
Like for instance this is chapter one's cover thumb nail and I just label the episodes (ex. Chapter ONE pg1-3)
While reading the episode thumbnail isn't really a deciding factor for me. If I clicked on the series I'm already committed to read at least some of it.
Although I have seen some people make their episode thumbnails make one bigger image when viewing the episode list, which is pretty cool.