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This is a very gray area, since it's mostly "fan art" and it's derivative work, distributed freely, without intent for profit. Sure you can pay someone to make fan art for you, but you can't usually sell it, since you don't own the IP rights. Also, you can't even really claim ownership of most this fan art, because it has no physical medium... It's not a painting, sculpture, written work.

Anyways, *many* copyright holders support fans and their communities, and even encourage the development of fan art.. Have art/costume contests, sponsor conventions, etc. Unless people are just making stuff for profit, and blatantly doing something against the spirit of the IP holder, I doubt they will even care.