Here's the artist that Joe Konrath uses:
http://extendedimagery.blogspot.com/ In his FAQ, he writes, Once you buy a cover from me, you own all the rights to the image, and can do whatever you'd like with it.
For Venom, I hired an artist listed on deviant-art.com and we agreed upon the usage rights. She gets to use it to promote her artwork, I get to use it to promote my book -- everyone's happy!
For Kinshield, I used another writer, Ronnell D. Porter.
http://www.wix.com/ronnelldporter/design - He does covers for $50.