Those in the Windows 8 news loop know that the current release of the operating system does not allow you to change the background of the Metro UI. This will be possible, however, in the beta version of the OS, scheduled to pop up in late February. Until then, if you want to change the green wallpaper behind the tiles in Windows 8, there is a tool called
Start Screen Editor that can do just that.
The application is extremely simple and allows you to modify the background panel either by changing its color or plastering a custom image on it. In order to apply a personalized image make sure you choose the PNG format and that the resolution of the photo is high enough to cover the entire screen.
Download Start Screen Editor for Windows 8 from this page