Took the logo from the website (that has a .mp3 that has been removed from the Portal 2 DLC - Community).
Two wallpapers, one is Black and White (as seen above) and the other is White and Black. Nothing special, about 30 seconds with The Gimp. May do better wallpapers with gradients, or some in-jokes between Portal and Half-Life.
Link to the site: [link]
(note: this is a fan-made site. But that was a REAL .mp3 sound file, now removed from the Portal 2 DLC)