The best way would be to have the creator/artist sign a waiver and transfer of rights. This is an agreement between you and them (it's about 1.5 - 3 pages long) in which they clearly state that they are transferring all the rights to you and waiving any future claims.
I advise having a lawyer draft this for you* (yes, I also draft these documents), or if you don't have any money, I am sure you can find some free or cheap templates online from one of the website that provide legal documents.
* the reason I advise using a lawyer/attorney is because this type of document isn't too expensive, and you need it to be tailored to your specific needs.
I've successfully helped over 350 entrepreneurs, startups and businesses, and I would be happy to help you. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david