Download key generator for Windows 8 Start Screen Customizer v1.0

The best thing of this tool is that it doesn't modify any system file to change the background image. It just paints the new background over the default background image.
Change Windows 8 Start Screen background image
Change number of rows of tiles in Windows 8 Start Screen
Customize transparency / opacity of Start Screen and tiles
Set current Desktop wallpaper as Start Screen background (New*)
Set multiple images slideshow as Start Screen background (New*)