This discussion kind of scared me, TBH I never really gave thumbnails too much thought. I know they are important in attracting new readers but I have always just cropped a square from the comic.
Now that I think about it, I still have my old thumbnail of Trivia from Nature cropped from the first episode, but the tone and focus of the series has shifted a tiny bit. But at the same time, below the thumbnail is always the title of the comic "Trivia from Nature" so I guess people make it out somehow that DRAGON + NATURE must mean some jokes from nature... at least I hope?
There was one episode thumbnail people seemed to like, with a puffer fish, should I change it to this?
I personally don't really mind when people use their best image for a thumbnail of their comic that's a B&W sketch, I can always close the window if I don't like the comic itself, like @blop said.
But it's worrying to see that a lot of readers care about it that it might even prevent them from clicking on suspiciously great thumbnails... This way creators using their thumbs like this hurt creators whose comic is the same quality as their thumbs 
