Metro Start wallpaper
The next choice on the Navigation Properties list says Show my desktop background on Start. If you check that box, a darkened and faded version of your old-fashioned desktop’s wallpaper appears as the background (sometimes called a “tattoo”) on your Metro Start screen
Microsoft says that’s supposed to make the transition between desktop and Metro less jarring. Personally, I don’t like it. I turn my Metro wallpaper into a solid, drab gray -- just to warn me quickly and visually if I’ve strayed to the Dark Side.
In general, you can’t use any old image as your Metro Start screen wallpaper: You’re stuck with the built-in options, plus a copy of your desktop wallpaper.