Microsoft qui a une page d'aide pour contourner le blocage de changement de fond d'écran, c'est rigolo ça :)

To resolve this issue, specify an Active Desktop wallpaper setting in Group Policy. To do this, follow these steps:

1. Click Start, click Run, and then type gpedit.msc.
2. Under Local Computer Policy, expand User Configuration, expand Administrative Templates, expand Desktop, and then click Active Desktop.
3. Double-click Active Desktop Wallpaper.
4. On the Setting tab, click Enabled, type the path to the desktop wallpaper that you want to use, and then click OK.