This sounds like a really fun idea. As @Sunnylady mentioned in her reply, I am not particularly competitive (except with myself) but I think this type of event could be really rewarding and allow us all to interact a bit more, which would be great, because the community here is the best of the internet, in my opinion.
I think secret voting is a nice idea, also perhaps different prize categories, if applicable, that could recognize various aspects of painting. I'm thinking things like: great brush work, adventurous spirit, breakthrough work, etc. Just thinking out loud.