That could be a good point but would have to do with the arrangement that Bill and Eric would have had. Did Eric have a contract and what was spelled out there.

Unless otherwise specified or verbally agreed to, I suppose, the artwork might be considered a "Work For Hire" and therefore owned by the person who "Commissioned" or paid for it's creation. Therefore it would be Bill's property unless he specifically had an arrangement where the artist retains all rights to materials that he created and the magazine had exclusive commercial use of it. It could be that Eric owns the art but has limited rights under non-competition...

That would be my understanding of it.