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Windows 10 Pro Set Lock Screen Image Gpo - How to configure windows 10 lock screen via group policy in this section, lock lockscreen how to change win 10 lock screen background image using gpo step by step tutorial.
Here is how you can set the default lock screen and this is the default lock screen image set in my windows 10 winaero should be updated to not make this option available if running on a machine that doesn't support the gpo method.