Check Registry
Computer\HKEY_CURRENT_USER\Software\Sysinternals\ZoomIt
Value: BreakBackgroundFile.
Also (copied from)
Computer\HKEY_USERS<your userid>\Software\Sysinternals\ZoomIt
Value: BreakBackgroundFile.
Set the file of your choice, full path.
Do not use double quotes.
You might consider to change permissions avoiding the system resetting the value when you exit Zoomit (remove full control to value, only allow read, and only allow FC to admins)