You'll find all my wallpapers in 1440p and 1600p (i'm not even sure that monitor size exists.. but it's still good for 16/10 monitors).
if you didn't yet, be my guest, take whatever you want There are more than 100 of them.
Just a though on 4k walls. It's a real problem for me, as with this kind of rez, you cannot protect your prints anymore, it'll be enough for most people to print anything with it. So 4k will be a problem for a lot of artists online.
4K would be a much better thread as it's clearly in the immediate future of computing. With the way Win 8 (and 10?) compress ever the living shit out wallpapers, you're much better off using 4k source images for everything anyways, even on 1080p and 1440p displays.