The web site 99designs is an absolutely wonderful place to solicit and receive all sorts of design submissions for a book cover project. I've used it twice now, and have been very satisfied with the results.
What an author does is pay money upfront (usually $295.00 for a decent contest) to 99designs. Then you describe what you want and your particular preferences and wait for the design submissions to roll in. It takes 2-3 days before you see much. After that, as designs are submitted, you may comment on them. Feeding off your comments, designers will often submit new versions of a design with changes as you suggested. At the end of seven days you declare a winner and the designer sends you the design in the format you need, and 99designs pays the designer. And no, you do not have to declare a winner. If you get nothing you like, 99 designs will give you a refund.
The only drawback, and this isn't really a drawback, is that some of the designers are not as connected as they could be. You could say, "this design is nice but it won't work because you have pictured an elegant slim lady and the book is about a fat sloppy lady." And so they put up a new design with some elements changed, but still an elegant slim lady. And you say..... and so it goes through many repeated fat sloppy comments and elegant slim re-submissions.