The WBC this year had a company that custom made gaming shirts. They had the WBC logo and in it you could put in 1, 2 or 3 game covers of your choice that were played at the convention.
Actually the last two years, the WBC did that. (The first year's the artwork was a bit faded when I got it, but last year's very nice.) The WBC shirts basically put the box artwork in the shield section of the WBC logo.
But the original poster is refering to T-shirts AH made (I don't know how long ago almost 20 years now) which simply had a picture of the box artwork. One of the players at Bay Area Up Front events usually wears his Panzerblitz shirt.