I needed a script to change desktop wallpaper for all users using the default one with an outdated company logo on. I came up with this startup script as a solution. Set $filesize to the size of the old wallpaper to only change wallpapers with same filesize. Use as logon script.

$filesize = 100593
$bg = Get-Item ("$env:USERPROFILE\appdata\Roaming\Microsoft\Windows\Themes\TranscodedWallpaper.jpg)
if ($bg.length -eq $filesize)
{
Copy-item "\\fileserver\UsrLogon\wallpaper\img0.jpg" "$env:USERPROFILE\appdata\Roaming\Microsoft\Windows\Themes\img0.jpg" -Force
Set-ItemProperty -path 'HKCU:\Control Panel\Desktop\' -name wallpaper -value "$env:USERPROFILE\appdata\Roaming\Microsoft\Windows\Themes\img0.jpg"
rundll32.exe user32.dll, UpdatePerUserSystemParameters
}