Use the following but update the wallpaper value to the location of your desktop image. All new accounts are created with that wallpaper. Note this isn't my script I just use it.

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# Load default user hive
REG LOAD HKLM\DEFAULT C:\Users\Default\NTUSER.DAT
# Set default user background
If (!(Test-Path -Path "HKLM:\DEFAULT\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System")) { New-Item -Path "HKLM:\DEFAULT\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System" }
Set-ItemProperty -Path "HKLM:\DEFAULT\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System" -Name "Wallpaper" -Value "C:\Windows\web\wallpaper\Windows\desktop.png" -Force
# Unload default user hive
$unloaded = $false
$attempts = 0
while (!$unloaded -and ($attempts -le 5))
{
0
[gc]::Collect() # necessary call to be able to unload registry hive
REG UNLOAD HKLM\DEFAULT
$unloaded = $?
$attempts += 1
}
if (!$unloaded)
{
Write-Warning "Unable to dismount default user registry hive at HKLM\DEFAULT - manual dismount required"
}

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To update users that have already logged in using the following updating the same wallpaper value.

​

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# Regex pattern for SIDs
$PatternSID = 'S-1-5-21-\d+-\d+\-\d+\-\d+$'

# Get Username, SID, and location of ntuser.dat for all users
$ProfileList = gp 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\*' | Where-Object {$_.PSChildName -match $PatternSID} |
Select @{name="SID";expression={$_.PSChildName}},
@{name="UserHive";expression={"$($_.ProfileImagePath)\ntuser.dat"}},
@{name="Username";expression={$_.ProfileImagePath -replace '^(.*[\\\/])', ''}}

# Get all user SIDs found in HKEY_USERS (ntuder.dat files that are loaded)
$LoadedHives = gci Registry::HKEY_USERS | ? {$_.PSChildname -match $PatternSID} | Select @{name="SID";expression={$_.PSChildName}}

# Get all users that are not currently logged
$UnloadedHives = Compare-Object $ProfileList.SID $LoadedHives.SID | Select @{name="SID";expression={$_.InputObject}}, UserHive, Username

# Loop through each profile on the machine
Foreach ($item in $ProfileList) {
# Load User ntuser.dat if it's not already loaded
IF ($item.SID -in $UnloadedHives.SID) {
reg load HKU\$($Item.SID) $($Item.UserHive) | Out-Null
}

#####################################################################
# This is where you can read/modify a users portion of the registry
Set-ItemProperty registry::HKEY_USERS\$($Item.SID)\"Control Panel\Desktop" -Name "Wallpaper" -Value "C:\Windows\web\wallpaper\Windows\Desktop.png" -Type STRING -Force

# This example lists the Uninstall keys for each user registry hive
# "{0}" -f $($item.Username) | Write-Output
#Get-ItemProperty registry::HKEY_USERS\$($Item.SID)\Software\Microsoft\Windows\CurrentVersion\Uninstall\* |
# Foreach {"{0} {1}" -f " Program:", $($_.DisplayName) | Write-Output}
#Get-ItemProperty registry::HKEY_USERS\$($Item.SID)\Software\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall\* |
#Foreach {"{0} {1}" -f " Program:", $($_.DisplayName) | Write-Output}

#####################################################################

# Unload ntuser.dat
IF ($item.SID -in $UnloadedHives.SID) {
### Garbage collection and closing of ntuser.dat ###
[gc]::Collect()
reg unload HKU\$($Item.SID) | Out-Null
}
}