There's another unrelated art contest online and the approach that they're taking is:
Find 3 external judges who don't know among themselves nor the participants, who are artists that have worked or are working in the industry, so they have the knowledge to take the best decision.
The prizes could be decided for categories: Grand prize(best overall), best art, best animation, honorable mentions etc.
They have allowed the participants to share their pieces online as much as possible, this is to encourage others to enter and promote their software, not necessarily to buy votes, though there's also a minor prize to the one who has the most votes, in fact. The prize for "most votes" is minimal so there is no incentive for rigging.