I’ve thought occasionally about prizes that you could offer which wouldn’t cost you cash but people would value. The first thing that crossed my mind was attending a photoshoot but I can see that would be impractical for most people, including me, and maybe you’re discreet about your location(?). Something that’s just this minute occurred to me is letting a winner describe a photo they’d like to see and you get it taken for them during another shoot so minimal extra cost. I suppose the problem would be if someone requested something inappropriate or infeasible.
The winner could be whoever is in the lead when we stop the dares, or if we never stop, whoever is in the lead on some significant date. Is there a St Venus Day? If you gave points for voting, you could give a prize to whoever gets to 20 or whatever and then reset all the counts.
Much simpler idea for a prize – could you use a high-resolution photo chosen by the winner from a gallery?