Based on some detective work using Process Monitor, Windows 10* stores custom lock screen backgrounds at:
Note: The last folder may have a different letter suffix, for example
You can determine your account SID by running the following from a Command Prompt:
wmic useraccount where name='%username%' get sid
SystemData and each of the folders below a protected with NTFS permissions that restrict even Administrative users from accessing them. To work around this do the following logged in with Administrative rights:
- In File Explorer type
C:\ProgramData\Microsoft\Windows in the address bar to open this folder.
- Right-click the
SystemData folder and choose Properties.
On the Security tab click Advanced
On the Permissions tab click Change.
Administrators in the text box and click OK.
Enable the Replace owner on subcontainers and objects checkbox and click OK twice to exit all of the dialogs.
SystemData folder. Click Continue at the permission error screen.
- Drill down to the folder containing your lock screen image, clicking Continue as required to get permissions.
*Confirmed on Windows 10 v1803