ok, here you go.. also by changing in the registry, the "stretch" mode can't be turned off.. more info
here.
• Start regedit.exe
• Goto "HKEY_CURRENT_USER\Control Panel\Desktop"
• Create a string value by the name: "WallpaperStyle"
• Set it to "2" for Stretch.
• Then you may have to log-off & on again.. after this, all wallpaper will be in stretch mode.
Other values: 1 - Center, 0 - Normal
btw, the normal caveat applies when dealing with the registry editor..