Ebook artwork has matured from its early days, where covers like these two could often be found:
Both are great stories, by the way, and The Dark Wing cover is for a release after Tor gave up the rights. For comparison, here's Tor's cover from the mid-2000s:
Now, let's take a look a sample of recent science fiction and fantasy ebooks:
The arms race of artwork for ebooks has elevated the covers to a level that can compete with those of traditional publishing. To stand out, an author now needs to place as much effort into cover art as the story. There are many sites that will provide technical advice for an author, and there are many debates over whether a portrait or an action scene is preferable. Meanwhile, tradpub is turning to the abstract:
If there is one bit of advice that I would give prospective authors, it would be to spend the time and money to find the art that best sells a story. Before a customer can read the blurb, the reviews, or the new featurettes, they have to have a reason to click on your page. An excellent cover gives them that reason. The challenge these days is to stand apart from the other excellent covers crowding the Amazon page. The bar has been raised.