Well, thats what Prae did, she offered 5 Ads for each selected picture. The other thing you have to bear in mind is that the server isn't making a profit off of the T-Shirts, unless of course they charge a little more (1-2 dollars more) per so that get a slight profit. Therefore, paying artists 5 Ads pear each drawing will mean a LOT of money to be handed out / distributed, for which there will be no returns.
Anyways, I just wish some artists drew the sprites for fun the self gratification of knowing that their drawings, art, and talent is being printed on someone else's T-shirt; but that isn't enough for some people aparently.