Just a bunch of random designs, right? Nope! They're all tied together in some way. BUT HOW??? That's for us to know and for you to find out. And then post about. And then we'll tell you if you get it. If you're first, you'll get a free copy of every tee in the sale! This is a thing referred to as a "game" in many cultures.
UPDATE: Solved by renfield19!