If you are struggling on how to change the Windows 8 Start Screen background image, here’s how to do it easily.
Click on the Windows logo key on your keyboard to call the Start Menu. Now type Personalize and choose Settings as in the picture below.
From the left pane choose any of the given settings:
Personalization screen will now open where you can change the color, customize your lock screen and notifications or customize your start and lock screens.
Click browse to select the desired image for the new background.