I can only speak as a Liquid Silver author, but when books go to contract, LSB gives its authors a questionaire for the cover, and the artists are open to receiving mock-ups of the cover, excerpts from the book, pictures used for the author's inspiration in creating characters/locations, etc.
I fell immediately and madly in love with my first cover, right off the bat, but other authors have asked for some changes and the artist worked with them until they were satisfied. One author ended up with a cover that was, well, interesting, and she got some grief for it, but it turned out that the feature generating grief from critics was a feature she herself had insisted on and the author believed the artist had executed it exactly as the author intended it to look.
Anyway, the cover artists at Liquid Silver (and presumably at other reputable publishers) really do want to create a cover that will pique interest, and they work with the writers.
JD
