To play an advocate of the devil, selling the gun is not illegal, though it could be argued that the process of printing the image could be. Consequently, I'm not sure your argument is really applicable.
That said, posting a notice that the customer is responsible for determining copyright ownership seems pretty reasonable.
I'm not sure that Wal-Mart is interested in investing the money it would take to train a minimum wage clerk to be able to professionally determine the artistic merit of a shot.
If it ain't watermarked, it should be free game according to the clerk.