couldn't change the lock screen image via Group Policy. There was also the issue mentioned below.
Microsoft recently release the November 2012 Cumulative Update for Windows 8/2012 that enables you to configure the default lock screen image for Windows 8 (See quote below).
You may have thought that this image was customisable by the users in the control panel already however this would only configure the image of the lock screen after the user had logged on to the computer. Meaning you were always presented with the Seattle Space Needle cartoon image every time you logged off or rebooted your computer. This image is nice to look at but this is definitely something the would be changed in most corporate environment to display their own corporate logo or a perhaps some disclaimer text. (Source)Quote:
Enable enterprise customers to customize the default lock screen.