The New Xbox Experience has its own themes called Premium Themes, but the old themes still work somewhat. And you can of course use your own picture as a background. Here are some of my NXE themes using Friends section as background.
Call of Duty: World at War Premium Theme.
Forza 2 - Peak Power (Premium) Theme.
Halo Wars You're in Command Theme.
Rock Band 2 Premium Theme.
Sonic Unleashed Premium Theme 1.
Viva Piñata: Trouble in Paradise Theme (Premium).