How To change Logon Screen on Window 8

NOTE: This option for using group policy is only available in the Windows 8 Pro and Windows 8 Enterprise editions.

1. From the Start screen, start typing gpedit.msc and press Enter when finished.

NOTE: You could also press Windows+R to open the Run dialog, type gpedit.msc, and  
          click/tap onOK.

2. If prompted by UAC, then click on Yes.

3. In the left pane, click/tap on to expand Computer Configuration, Administrative 
     Templates,Control Panel, and open Personalization. (see screenshot below)

4. In the right pane of Personalization, double click/tap on Prevent changing lock screen image.

    (see screenshot above)
5. Do step 6 or 7 below for what you would like to do.

6. To Enable Changing the Lock Screen Background Image

    A) Select (dot) Disabled or Not Configured, click/tap on OK, and go to step 8 below.
          (see screenshot below step 7A)
    NOTE: Not Configured is the default setting.

7. To Disable Changing the Lock Screen Background Image

    A) Select (dot) Enabled, click/tap on OK, and go to step 8 below. (see screenshot below)

8. You can now close the Local Group Policy Editor window if you like.