I recommend: Open prompt for suggestions, lean towards popular ones, but ultimately just use your judgment and pick one. That's simpler, faster, and more likely to include quirky/clever picks, instead of literally "the most popular thing every time."
We'll just have to hope the people who decide you have shit taste will tell you instead of bailing quietly to avoid a confrontation.
(Voting won't actually help that aspect, btw. An open election might just settle on Worm over and over, steadily driving out everyone who doesn't agree with the majority)
Why not do it like they do in The Voice? Assuming you get enough entries, anyway. But if you do...have the forum members vote for, say, the top ten of each genre or theme. Then, once the whole thing has been narrowed down, it goes to the regular judges, who start judging each picture by technical proficiency, creativity, and so on and so forth. Then, once the category winners are decided, open it up to popular voting again to choose the best overall (in which case, the runner up for that category would take that category).
And if you end up with the same people winning over and over again, add a new rule saying that any contest winners from the last contest cannot enter the current contest. That way, it will be impossible for any one person to really dominate the contests, and encourage new talent to keep applying.