user wallpaper is in a different location now
You need to go to the User Wallpaper folder in themes and right click wallpaper.jpg and click Edit Link.
/var/mobile/Library/SpringBoard/HomeBackground.jpg
is the link
Then change Wallpaper.jpg to [email protected] (for iphone 4 users)

Same goes for the lock screen theme but it's
/var/mobile/Library/SpringBoard/LockBackground.jpg
I'm not sure about adding the @2x but I imagine you'd have to.

respring or reboot.