We had a photo event here before but most members at the time wanted it to be non-competitive due to hurting peoples feeling (friends do not compete against each other) In my opinion friendly competition can help people try harder.
One thing I see necessary is that people who vote do not know who the picture is from as there will be too much of a desire to chose on who it is by as apposed to the picture itself.
The other is that people with a photoshop knowledge have an unfair advantage over people who shoot all natural, in nature or in a studio setting.
I think categories could help seperate them out