
Understand that the moment anything is created whether it be written or drawn, the creator owns the copyright — that's the law.
Copyright can only be transferred in a written legally binding agreement signed by the creator of the work stating they are transferring their rights to you.
Saying you own it because you paid for it doesn't make it legal fact. If you do not have a written agreement from the creator of the artwork, specifically transferring the copyright to you, you do not own the copyright to those graphics.