The upcoming beta release of Windows 8 will allow users to use more customization features but won't allow users to use their own picture as the background of the Metro Start Screen.



From Neowin:

So what can you do with the Start Screen background in Metro on Windows 8? As you can see in the picture above, You can choose from eight different styles of background in the beta version and then alter the color of that style via a color scroll bar. The colors are also very specific to avoid users choosing strong colors that would obscure text.

The article also says that the beta version will allow mouse users to have more control of the Metro user interface. You will be able to use your mouse's scroll wheel to move the Start Screen's interface from side to side, among other features.

  Windows 8 won't have Start Screen picture customization