As others have said, pretty concisely, the only way to judge is to actually judge. Relying on RNG is lazy and stupid; it's literally brainless (even though you fear to judge, we can't help seeing good art vs bad and making the comparison ourselves, even if we don't voice it every time, though you'll probably hear some pent up resentment bubble up when bad art wins). You keep insisting that it's not a contest but that's just hiding behind semantics when you have such clear winners and losers.
if you insist on despised RNG - then get rid of its obvious pitfalls. Make it a streaming event or an in-game event and make it as transparent as you can, assign tickets on-camera and pull winners out of a hat or get in game and use /dice or something. Interact with us. Don't decide it all out of sight and then expect us to take your word it was all 100% above board and not shady at all. Also, consider having a fluid number of winners - if 50 screens are submitted one month then you should have more prizes available than months where you only received 5 entries.
If you insist on 'it's not a contest' - remove prizes for winners or revert to participation prizes for all. The old creative convention thing wasn't too bad except the green hat encouraged entries motivated by greed. Making the green hat untradeable/ statless/ homogenous would've patched that hole and slowed the sink. All you've done here is maybe reward participation whilst passively demoting effort and talent (like every time a non-artist declares artists are unfairly rewarded in these threads).
If you insist on rewarding some (winners) and not others (losers) - grow some responsibility and judge the winners. Have the decency to honestly look at our submissions and give an honest opinion, just the thing you're asking of me now. Even a bad decision is more acceptable than no decision (see how many bad responses you got for the cosplay contest? None that I saw; maybe you removed them but I doubt it - the whole thread had a nicer vibe throughout).
Others have voiced concerns about commissions. I, personally, don't care if the art is submitted by artist or buyer so long as consent is sought and given.
Ultimately, you need to decide what you want these loading screens for. If you don't want this stress then just choose some nice official arts and leave them there till the heat death of the ROVerse. Also consider that most machines, including my 8+ year old machine, will load maps almost instantly. Loading screens might not even be necessary.
If you do want user screens then ask yourself why? To make the game look nice? To promote the game? Reward player fandom? Pick a mission and choose actions that help it, drop actions that hinder.
Keep us enthused.
"Fan art; art made by enthusiasts of its subject matter." -Wikipedia.