@Jelw, gyaah! Thanks for using my thumbnail as an example! XD;
For those who are working on long form comics, I think it's a good idea to treat the thumbnail as a simplified version of the front cover of chapter 1 or volume 1--- to avoid spoilers. In my case, I ain't gonna show the skeleton dude in his full pre-skeletonised self right off the bat, lol. If your comic has a love triangle, even if you've already decided on the one true pairing in your mind, don't show them together on the thumbnail! I think it's perfectly fine to tease the readers (new and old) a bit. But don't slap on anything that's irrelevant.
Use whatever attractive elements you have in your comic to your advantage. UH relies quite a bit on wtf moments, so having a skeleton in the thumbnail doesn't hurt and hopefully when people see it, they go "wtf?" and click, lol. The art in the thumbnail was originally printed on a manga magazine cover for school, so I put various thoughts into it for it to represent my comic. I made sure that the girl's hands are shown in the thumbnail for added expression of (hopefully funnily exaggerated) horror because there's comedy in the story.
@Iron's Otter Kind of Comic's thumbnail is pretty awesome and quite suitable for a cute, silly and sometimes weird comic strip series. Look at the thumbnail. An adorable otter in a psychedelic pattern and colours. And it's all strangely relevant to the comic. It's easy enough on the eyes and it makes you wanna click! XD
As long as the thumbnail somehow represents the overall feel of the comic and gives the readers expectations that can be fulfilled in the actual content, then it should be alright 