As a general rule, when an artistic or literary work is created, the author is the one that holds the copyright. Nobody can copy, distribute, or display the work without the author’s permission. When an painting is sold, the buyer owns the painting itself, but does not have the right to use that image for anything else other than to hang it. But what happens if the artist was hired specifically to create that painting for the cover of an upcoming novel? In some cases, the copyright is owned by the buyer, but only under very specific conditions.
Read full article: Who Owns Works Created by Freelancers?Art Law Journal.
artnxtleveljournal.com 
I would say that the ones who publish the novel should have established specific conditions with the artist. Buyers should not use the image, but I’m afraid that is not 100% accurate. I think there are some who use them for different purposes, as well.