I think a solution could be everyone votes for the best frames.
Then those artists are called back to work on the frames people made bad.
Then those artists are called back to work on the frames people made bad.
You give several people the same frame to draw, then you just have everyone pick their favourite. The one with the least votes can be temporarily shadow banned from their next drawing appearing as an option. Alternatively, you can reward the one with the most votes.
But honestly, maybe just allowing people to choose the frame they like might be enough and you don't even need to do much more.