I'm going to settings then wallpaper but it only changes the lockscreen background.... is there another place?
If I remember from watching the keynote, if you go to the settings for wallpaper it should come up asking to change homescreen, change lockscreen, or change both ...
Seriously, if they don't allow a measly image of that resolution to be applied in the background behind your apps, then they're ripping a lot of people off by taking that feature off them.
It's a picture, it's a couple of MB, if that ... it's hardly going to slow the phone down that much, if at all. Surely?