Using powershell
Option 1
Get-ItemPropertyValue 'Registry::HKEY_CURRENT_USER\Control Panel\Desktop' -Name WallPaper
Option 2
$TIC=(Get-ItemProperty 'HKCU:\Control Panel\Desktop' TranscodedImageCache -ErrorAction Stop).TranscodedImageCache
[System.Text.Encoding]::Unicode.GetString($TIC) -replace '(.+)([A-Z]:[0-9a-zA-Z\\])+','$2'
SRC: https://www.thewindowsclub.com/how-to-find-the-current-desktop-background-image-in-windows