oh, and by the way - just so you know - its is common for the artist to retain copyright where as the "customer" gets "right-to-use" (not sure what its called in english). But of course possible to write in a contract that you get "exclusive use" of the artwork, to make sure it isn't used anywhere else. Normally you will also have to check with the artist if you want to use the artwork for another project than the one it was commisioned for so he/she can make sure that it isn't used for anything they wouldn't sanction. (like hate pages or who knows what).

Best of luck with your project