automatic wallpaper swapper

hi, i use portable apps on my usb stick.
and i want that the wallpaper i have putted on my usb stick become automaticly my desktop wallpaper, and this works at home, but if i stick my usb stick in any computer at school and i start portable apps, the desktop wallpaper changes but we have an active desktop at school so the wallpaper comes under the active wallpaper and so cant i see the wallpaper i want to see.

is there any way to let my wallpaper overlay the active desktop so that i can use my personal wallpaper at school? (we have no accounts at school), every setting you make at any pc at our school gets automaticly deleted if you logg out, so i use my usb stick as my user profile

thanks in advance
Davy