We already have a sort of monthly photo contest that does not have a lot of participants. If we start charging $$$$ to enter the contest, I would imagine the participation would drop. I already donate money to the site, why would I donate to submit a pic in a contest.
The t-shirt to the winner would not be bad idea, but at this point, if you want a GMTN t-shirt you have to buy it yourself from a 3rd party. Someone would have to spend their own money to buy the t-shirts to give away. Unless the money from the buy in was used to pay for the t-shirt, but then you are taking money from the winner and maybe they do not want a t-shirt.