This will set the default wallpaper for all users who login to a Windows 7 machine.

First, login to the machine with an Administrator level account.

Open up regedit.

Load the Default profile ntuser.dat. (see http://jim-zimmerman.com/?p=196)

I loaded the hive using JGZ for the name. Navigate to:

[HKEY_USERS\JGZ\Software\Microsoft\Windows\CurrentVersion\Policies]

Note: If you do not have a System key under Policies, create one now.

[HKEY_USERS\JGZ\Software\Microsoft\Windows\CurrentVersion\Policies\System]

Then, in the System key, add the following two String Values:

“Wallpaper”=”C:\\Windows\\Web\\Wallpaper\\mydirectory\\myimage”
“WallpaperStyle”=”4”

Note: I put my wallpaper in a subdirectory I created in the Wallpaper directory.

Unload the Default hive, and reboot.