How can I change the lock screen in Windows 8 that appears for the default user?

Solution 1:

MS released an update to allow this without any hacks:

Win8: How to Manage the Lock Screen Image on Windows 8 and Windows Server 2012 http://support.microsoft.com/kb/2787100/en-us

The update “Windows 8 and Windows Server 2012 cumulative update: November 2012” adds functionality to the Control Panel group polices that allow an administrator to designate a lock screen image on their Windows 8 and Windows 2012 computers. This setting lets you specify the default lock screen image shown when no user is signed in, and also sets the specified images as the default for all users (it replaces the inbox default image) Some restriction apply. See the Restrictions section below.

The new group policy is named “Force a specific default lock screen image” and can be found in this path in the group policy editor: “Computer Configuration\Policies\Administrative Templates\Control Panel\Personalization”

Solution 2:

Found it.

c:\ProgramData\Microsoft\Windows\SystemData\S-1-5-18\ReadOnly\LockScreen_Z\

Replace the picture in there with your custom background picture. It must match the name of the current background picture (for me, it's LockScreen___1680_1050.jpg). Logoff (or restart) to see the result.

NOTE You may have to take ownership (or grant yourself permission) to view the contents of the SystemData folder and it's subfolders.

Credit goes to someone named Brian Michael Reed posting here: http://social.technet.microsoft.com/Forums/en-GB/w8itprogeneral/thread/82e7ae0f-1b6e-4d04-ae05-8278911af3e2