There are four ways that a wallpaper will, or won't make it onto the site.
1. The author is a long-time, respected contributor to the site, and the skinning community in general, and has direct access without 'moderation', because he/she is 'trusted' not to upload inappropriate content ['borrowed' copyrighted works, porn, etc]
2. A wall goes through moderation, and is so mind-numbingly horrendous, or it is porn, or it's Britney Spears rendition xyz123, that it gets an instant 'kick' by the first moderator who has the misfortune to see it.
3. A wall goes through moderation, and is so mind-numbingly awe-inspiring that the first moderator who sees it, approves it.
But mostly...
4. A wall goes through moderation that looks like the guy's last 10 submissions but, oh, it may be better, maybe not....and it receives a 'comment' by the first Moderator...then another may look at it and doubt its origins...[looks like an'adni', but recoloured] [ isn't that Misery's-Deaddreamer, but with a swirl filter?]..posts a comment.
Perhaps a third will check...yep, seen that graphic in the Nijikon wall...kick it...or is it fair-use?
Kick it...no permission.txt.
So...we kick it, with an email saying 'moderated...permission required from original author'..
Response?...'Accusing me of ripping, you effing pricks..if you'd 'bothered' to go ask him he'd have told you he let me.
Stuff you, I'm gonna go to Devart where my work will be appreciated...stick it up your arse you bunch of t**ds'
Polite email response to outburst...
'your loss, not ours...BTW...many of the same moderators here are closely involved with Devart, and the other skin sites...if it's an issue here we can make sure it's an issue there as well...same reason.'
OK, so that got a bit 'specific' but perhaps you get the mechanism.
When a wallpaper may take 3 or 4 separate, individual human-intervention processes, each, and we get, 0oh, one or two walls a day [I wish], then you'll understand it's not quite as arbitrary as you might think.
Sure, some '[relative] crap' gets through, but you should just see what hits the cutting room floor...in droves....