Did you think I was going to get off the topic? You don't know me very well!
You can't read a book by the cover but much of the time books are bought because of their cover. If a reader has an enormous choice, a good cover will catch their eye and they'll investigate. Perhaps the book won't interest them even if the cover art does. But maybe it will lead to a sale.
So now we have a situation where instead of a smallish cover in a bookstore, we have a thumbnail. How is one to convey something crucial about the book when the image is barely large enough to be seen?
One needs to have a simple, uncluttered design with easily to understand images and easy to read type. That probably is in the realm of personal opinion to a great extent but most of us understand what that means.
Here's an image I worked on this morning. I am looking for a replacement for That's Amore. Actually I would like to retitle it while I'm at it but I don't know how that impacts the situation at Smashwords. While the Brooklyn Bridge has a lot of details, it seems to just read big city bridge to me. Of course, the drawback, as opposed to drawbridge, of this image is that it's oriented horizontally instead of vertically. If it's made smaller to fit into 990 pixels wide, then there will be a substantial area below it that needs something else. Or something above it.
It could be treated as a postcard on a page/background. It requires time to shuffle the elements of a cover on a page and see what happens.