I have one of the books on that site (the Mike Resnick, a couple of pages in.) It does have a bad cover, but it has an author's name I recognise, it was free, and other people recommended it.
The cover is not of paramount importance, but if I'm browsing a list of books, it's quite likely that I'll only have the title, author and cover image to go from, and I have to triage somehow.
The title is probably at least as important, and it's something I'm struggling with for my own current work-in-progress (and I can't do a proper cover without it). The working title is too vague, and some meaningful alternatives I've come up with give the wrong impression. And okay, I still need to work on the words inside a bit, too.