There will be three different categories in the contest - cookies, cakes, and original desserts. Five winners in each category will be picked on December 1st and the 15 winners will be announced on December 3rd. You do need to hold a sign in the picture with your Zynga forum name so that they know that it was actually you, and no Photoshopping will be allowed whatsoever.
This is a neat contest, because players have already made some fantastic looking desserts in the past and there will be lots of great ones to look at.
View the officlal thread here for all the details and to see the submissions so far.