I changed my wallpaper on my new MacBook Pro and now it always reverts. When I reboot it goes back to this one no matter what I do.
That's a known bug, many have the same issue.
But, did you already update to 10.8.3 which might resolve the issue?
Also, try repairing permissions.
I haven't read them all, but you may find some useful information in one of the several threads that have been posted on this problem, such as these:
wallpaper changes to default after restart
Wallpaper wont stay, reverts back
MacBook Pro Retina changes wallpaper back to a apple default one at start up.
rMBP reverting to old wallpaper most times I reboot.
Wallpaper keeps going back to default after reboot