Posters, t-shirts, and many other items that you can sell online will require a custom graphic that you’ll place on the item. You may also want custom branded items with your company log.
You can easily and inexpensively have your graphic design needs created by an online freelancer. But it is critical that you own all the rights to your graphic designs.
Below are two good resources on the topic.
Who Owns the Rights to Your Design Work? August 27, 2014
How to make sure you actually own the creative work outsourced to your graphic designer by Creative Genius Law – strategic legal partners for creative industries.