Yeah, covers are nice for when you want to print, or for other sites like RoyalRoad, but here on Tapas the main focus is the thumbnail.
But regardless, it's really a marketing ploy while trying to stay somewhat true to your story.
I create all the artwork in my covers/thumbnails/banners, since I'm primarily an illustrator/comicker.
This is my current thumbnail. I intended it to be a bit shojo-y, to reel in the large audience on Tap that is used to that sort of thing, and it seems to be working, I've noticed a steady positive sub-rate.

But the thumbnail before that was this.

I rarely digitally paint so I really like the effort I put into this. I still plan on using the main image in my novel's cover once I properly release it on Amazon when it's complete. I do currently have it as the banner though.
And like I said before, a large part of Tapas' readers are quite used to comics, particularly webtoons/manga/anime style, so you'll see a lot of that style in the thumbnails. I've also found a lot of stories are influenced by those very things too, so it makes sense for the authors to choose art that fall into the same style.