Patchou,
First of all, great idea for a competition, I think it's pretty innovative that you've used the game as a way to advertise plus!
- and also to give back to the community!
Secondly, a general 'human behaviour' question:
Are you keeping a log of where/when pixels were clicked? I'd be very interested in seeing the results after the competition has ended (I'm sure lots of others would be too), just to see how much the different parts of the image influenced where people clicked.
Maybe just a 'temperature map' sort of thing, with hotter colours showing where the highest number of clicks were?
Cheers
Martyn