Is there any registry tweak or any tool which can be utilized for changing the Windows 8 Start Screen background? I can see only few options to customize the Start Screen of Windows 8.
Is it possible to place a custom image as the background?
In Windows 8.1 you are able to set the background of your Start Screen to match the desktop background. You can get to this setting one of two ways.
From the Start Screen, press Win+C to open the charms bar, and then click Settings. Then click Personalize. In the list of background options, the bottom right will set it to your desktop background.
From the desktop, right-click the taskbar and select Properties. Click on the Navigation tab, and under Start Screen you will see "Show my desktop background on Start". Check the box next to it and press OK to set that.
If you want to set it to any arbitrary image, I know of no way to do this with native Windows settings. However, 3rd party solutions exist. One that I know of is Decor8 by Stardock or Start Screen Customizer. It provides a lot of options for customizing the look of the Start screen, including setting it to an arbitrary background image of your choosing.