Actually i don't see the need to bother with this wallpaper copy paste as mywall.png from the script.

as soon as you choose a picture for wallpaper it will copy a cropped image in /.wallpaper folder. so if you create a 854x854 pic and call it wallpaper.png in this folder you are good to go. the only problem i see there is if you have a centered logo. it will be deported to the side no matter what. and i don't know why...