I was on a tropical fish forum once (before they all closed) and we had a monthly photo competition. A thread would be started titled 'November Photo comp' (for instance), laying out the theme for that month. All entries would be sent by pm to a moderator and once the deadline (3 weeks) had passed the moderator would post all the photos in the thread with a poll and members would be given a week to cast their vote for which one they thought was the best photo. Then the winner (plus 2nd & 3rd places) were announced and the winner picked the theme for the next month's competition, which was announced in a new thread, so there was one thread for each month. That worked ok.