psychological subjection, if theymos makes a pizza, many will be tempted to vote for theymos' pizza even if it's not the most beautiful pizza
to have voters who vote objectively, in my opinion, we need to publish the pizzas anonymously so people can really vote for the one they like
Someone can argue it doesn't have to be beautiful, what matter is the taste (even though they didn't even take a bite) to have voters who vote objectively, in my opinion, we need to publish the pizzas anonymously so people can really vote for the one they like
But I understand with your point and you're correct, however the rules say you need to post your username + date on your pizza, even it's not posted by the user, people can still know who make it.
If people don't mind with the hassle, anyone who want to join need to post in this thread, then @OP or someone else could message each user about the special code that need to be written on a piece of paper, then post it along with the pizza. So, this could prove the authenticity of the participant than just uploading the pizza without any proof.
Although, someone can still abuse it by sending personal messages to his friends/followers/gangs in social medias to vote his pizza.
Therefore, specifically stated conditions are the key to the transparency of the competition. 
