90% sure Citadel/Games Workshop have the Reproduction rights. If it was a commission by them (as in they got him to paint the work FOR them), they would have it.
IF it was a piece of work done which they then BROUGH for use (like buying the rights to use it) then they would probably be for a limited time.
IF they were a smaller company or more modern, then the Artist would have rights.
In fact, these artist rights issues were one of the reasons that we lost the rest of Kaleb Daark (boy, I would love to know if the next part had atleast the story brief made... mostly likely it did) GW people were used to them having ownership, Wagner and Grant were big enough comic people to have 'we keep the rights' (in British comics, this is still rare with big companies. Even if you create something new for say the Beano, you don't get the rights.. or even really re-print fees. But if you are a big 'star' creator, you can keep rights and some smaller companies and stuff will do stuff that allows you to own the rights.)
