Who owns the rights....?

In fact, the artist would have to sign the rights over.

There are many types of rights, including "first printing" rights, "First North American rights", "First world rights", as well as "digital rights" and so forth.

It's impossible to predict what the artist will do -- depends on how knowledeable he is about subsidiary rights.

And no printing company will touch the work for reproduction without a signed statement saying that you've acquired the rights.