I was in a similar situation once. Kind of. Back in my freshman year, I was making cards for the holidays on the computer and selling them to everyone. My gym teacher wanted one advertising the school's wrestling team (since he was the coach) and told me specifically what he wanted on the card. Well, I made the card and he was so impressed with it, he turned it into the lady at the school's bookstore to have her make T-shirts of it. She did so and they sold out immediately. After that, I won the school's biggest award (The 'You Make It Happen Award', which is very, very hard for a student to obtain). I was actually pretty popular the following week I won that award, too. Other than that, that school year flat out sucked.
Yeah, I heard you can do that kind of stuff on cafepress. But I think if you want something done really well with it, it's best to use a program like Paint Shop Pro or the like.