How to override the default Windows 10 lock screen image for all users?

Question

I would like to change the default Windows 10 lock screen image file shown below to something else.

Solution

For Windows 8 and 10, the default lock screen image is located in C:\ProgramData\Microsoft\Windows\SystemData\S-1-5-18\ReadOnly\LockScreen_Z. In order to access that folder, you must first take ownership of the SystemData folder (or set its ownership to the the Administrators group).

LockScreen_Z contains multiple images of different sizes, and the one that matches the screen resolution the closest will be the one that is displayed (most likely LockScreen___1920_1080_notdimmed.jpg). Replace the file with your desired image of identical size.

Note that this only works for users who have not customized their lock screen image through the Windows Settings, or if lock screen customization has been disabled by a network administrator.