little dachshund cartoon below, which I sell on T-shirt, etc., at CafePress.
Now don't misunderstand me, I'm not in any kind of position to grant copyright. That's done by the United States Copyright Office. And it requires submitting forms, etc., none of which I have ever actually done. But if you're about to invest in getting some artwork printed, especially to be sold on the internet, it's a good thing to add to your *cost of doing business*. As the creator of the dachshund drawing, I hold copyright, but I really don't think that I have enough documentation to take it to court, although I might try. So I run the risk of someone stealing that cartoon. CafePress isn't my main business, anyway.
As an artist who does custom illustrations, I have to admit to losing a fair amount of illustration business when the internet was new. It was a free-for-all, with apparently no rules, so if someone needed a cartoon of, for example, a dachshund, they just went and found one, and used it without permission. But lately people are getting caught.
So, here is what I suggest - Hire someone (like me) to create original art for you. Get a written statement from the artist giving you exclusive rights to the artwork (don't assume it) and then get it copyrighted. If anyone challenges you, you have the documentation to make them go away. And if anyone steals your copyrighted image, that same documentation is your ticket to court. Sue them.
Copyright simply means the right to copy. And it's enforced by law. Please don't listen to my advice, talk to a lawyer. If you're getting ready to spend thousands of dollars to have something printed, my small fee for creating original artwork, and the lawyer's fee (which is probably much higher) is a small price to pay.