Oh easy
not judging but voting.
I'll declare it closed at the end of the month and then anyone but especially the people that have nominated, then get 3 votes. So even if they have made all the 4 nominations allowed and they insist on voting for all their nominated images, they will still have to ditch one. Of course someone else may have nominated something they fancy more and they will vote accordingly.
1st vote 5 points
2nd vote 3 points
3rd vote 1 point
then after a set period, maybe a week I'll add up the votes and declare a winner.........phew!! It does mean a fair amount of participation but hey, why are we here.
Now pick holes in my theory