Mac: If you're not a big fan of the grey fabric background that lines Mission Control (and some other parts of OS X Lion), our favourite hackintosher tonymacx86 has figured out how to change it and it's pretty easy. Here's how to do it.
Not-so-deep down in your system hides CoreServices, where many of the invisible apps of OS X are kept. To change the background, we just need to get to Dock.app. You can find it here:
Right-click it and choose Show Package Contents. Then you'll need to drill down a little further to find the default background image:
Just rename that file to defaultdesktop-old.png and save another 256x256 patterned image called defaultdesktop.png in its place. If you'd prefer, however, to use a non-repeating image and a proper desktop wallpaper, you can do that as well. Just rename defaultdesktop.png but don't replace it. Instead head back into the CoreServices directory (where you found Dock.app) and replace DefaultDesktop.jpg with one of your choice. You can also replace the Dashboard background by replacing the file named pirelli.jpg. Pretty neat!
Change the Mission Control Background in Lion [Tonymacx86 Blog]