By default, in Windows 8, users can change the background image shown when the machine is locked. How to Prevents users from changing the background image shown when the machine is locked?
Open Local Group Policy Editor
Press Win+R key combination to launch Run, type gpedit.msc and press Enter key. It'll open the Local Group Policy Editor.
Expand the Personalization configuration item
Expand to the Local Computer Policy > Computer Configuration > Administrative Templates > Control Panel > Personalization configuration item.
Change Prevent changing lock screen image configuration
In the right pane, double click Prevent changing lock screen image item.
Select the "Prevent revent changing lock screen image" dialog box.
If you enable this setting, the user will not be able to change their lock screen image, and they will instead see the image set prior to enabling this setting.
Click "Enabled", then click Apply, and then click OK.
Force Group Policy Updates to take effect
Press Win+R key combination to launch Run, type gpupdate /force and press Enter key.
And now you have enable or disable "Prevent changing lock screen image" in Windows 8.