Remember that this is the internet and this forum is full of hueknights. If its 100% up to the users to deside we will just end up with stick figures with popolion faces winning.
Instead of submitting the art to the forum, have us send it to you on email.
Make one forumpost with all the artwork that follow the rules of the contest, no 5min paint drawings, no lewd stuff and that it actually follows the theme of the contest, and let them be anonymous when you post them.
When you deside who is going to win, let 5 of them be the staff favorites, 5 be user vote, and let 10-20 be “the goddess choice” AKA random. Mix all of them up and use 10 of them as loadingscreens that you rotate out every week. Prizes will only go to the 10 that got picked by the staff/voters, but alot of the others will get their honorable mentions and maybe a tiny ingame prize. A gatcha cube seems nicely fitting.
This way everyone wins. The staff gets to sort out all the stuff that is bad, the users get their favorites to win and everyone gets to see a lot of nice new art every week.