According to international copyright law, as long as you can prove that you're the first to come up with "work that can be protected," you have copyright.
So you don't have to have that C with a circle around it to tell the world you copyrighted.
If you're the first person to come up with that creative work and use Ethereum's blockchain to turn it into an NFT, then you're the copyright holder.
If you create a picture and sell the rights to somebody else, and they turn it into an NFT, they are the copyright holder.
Based on the above, your plan of using NFT images for POD gear is a non-starter.
Try to get permission or come up with your own designs.