I think it's great idea. I would at least try it. I'm not really a T-shirt person but I know many who wear them and I think some would be interested in something like this.

I like your landing page as potential wearing customer, but I don't think it would attract me to advertise with you. I would only change the pictures representing what kind of pictures people upload. Here you can't see that person wears T-shirt on an event or in a day in their life, you only see T-shirt and don't get the feeling person is wering it outside of their bedroom.

I think that feature of choosing what brand will you promote with this T-shirt is important. You can see that many commenters saw this as a problem. But I think that here is a problem from the side of companies who would advertise on your T-shirts as well. If I used your service for promotional purposes, I would like to know that in front side of T-shirt there would not be something opposite of what I would like my brand to be. But I think that for starters you can filter that out manually.