I have no idea what i-shirt is but it's likely that if an user uses copyrighted material for his i-shirt, you'll have to pay because it's your platform. Bonus points for not using a censorship filter in trying to do this.
Ask him again about censorship in the EU when his most popular shirt is "Hitler did nothing wrong" or some other such nonsense.
No UGC will be allowed and the text you add will not be displayed anywhere :)
Try an i-Twitter or an i-Facebook.
very innovative. meanwhile this will likely cost a lot to qwertee