One of the most important feature in Windows 8 is new Start Screen, Microsoft has removed Start button and introduced Start Screen, Windows allows you to change color scheme of background but there is no option to change background wallpaper.
Windows 8 Start Screen Customizer is a tool which allows you to set a slideshow as your Start Screen background. You can select multiple images or select a folder which has multiple images, and then select the time interval for it and you are done.
You can set your desktop wallpaper as background picture (not support for multimonitor, it just uses the main monitor’s desktop wallpaper)
You can select your pictures from any folder and set it as a sequence of background pictures.
There is a blur effect available, it is slow and slower on bigger resolutions, but it might work for you.
Tiles opacity can now be changed (not perfect though)