My first idea was to choose the image I liked best (like badcop's richbitch last month). Then, after some discussion via PMs, it expanded to voting for Image of the Month. For ease of use I limited the choices to 5 winners with the most votes. Granted, the 'most votes' criteria means almost nothing - voting fluctuates day to day and a gorgeous image (like Serps' Mighty Steed) may win with less (than the most in the month) votes, this very much applies to all anonymous contests.
All that said, I don't have a perfect solution. Maybe all of the 30/31 winners should qualify (don't know if a 30 question poll will work). Or maybe I should start the thread with a request for nominations first and then use the results to set up the poll/voting.
I'm open to suggestions.