Portal 2 menu background?

think you have to beat the game again

Have you tried playing through the last chapter again?

Beat the single player again

You can load the final map and re-beat the boss fight. Then just before the ending, save the game, so you can load the game later.

you can load the last level, or load the last checkpoint to the Wheatley Lair part..

Custom GUI? =/

Which background you get is based on which chapter you played last. If you replay the ending, you will get the floating background.

At the small cost of processing power you could have it as a desktop background if you like it so much.

Load the last chapter/checkpoint and beat the last boss (again) ^_^

Spaaaaaaaaace!! (:D)