If your work shows up on a product sold by somebody else, what do you do? First, call a lawyer, duh. And do not sign anything that anyone puts in front of you until you do.
Memoirs.com has clearly taken the very cool work of Kyle Theobald (http://www.creativecenter.edu/portfolios/Theobald_Kyle/) and made shirts that they’re selling online. See below for the obvious copying.
Original Work by Theobald: