Voting is subject to fraud, as has been shown in the past, when entrants ask all their friends to vote for them. This means that the best image doesn't win in such a popularity contest, but rather the image whose creator has the most friends. I didn't read their rules, but these contests are often judged by a panel of employees, and the judging process is usually completely opaque. If we need to know their criteria, they'll probably post it somewhere.