I'd say more creativity would be needed than for the T-shirt contest. The product itself - stickers is not the best immediate tool for promotion, but could be used to liven up the atmosphere in the Tg group and of course to leave some Marks on any 'digital wall' of the Web.

What about similar prizes as for T-shirts with one difference - splitting the awards like this :

• 2 x 2000 $CTM for 2 1st places
• 2 x 1000 $CTM for 2 2nd places
• 2 x 500 $CTM for 2 3rd places
• 250 $CTM for every participant