PNG??? 24 bit transparency??? Sketches wont work??? Scan into photoshop??? This is why only 2 people have entered.
heh, do you want to enter the design on a napkin? Sorry but the technical stuff is as important as the idea. Otherwise you would get a blob of mud on your t-shirt. Quality is important.
There are step-by-step how-tos on the cafepress links I posted if you want to format your files. If you have any questions, ask me. Happy to help.