If you wish to change lock screen image in Windows 8.1 (this should work with Windows 8 as well), follow these steps –
On start screen, search for GPEDIT.MSC to open Local Group Policy Editor.
Go to Computer Configuration\Administrative Templates\Control Panel\Personalization and open setting “Force a specific default lock screen image”.
Select “Enable” option and enter path to the image file to display on lock screen.
Click on Apply to apply the changes. Next time you restart your computer, lock screen will display new image.