This will show you how to change the background wallpaper of mission control in osx lion and mountain lion.
- open finder
- goto /System/Library/CoreServices/Dock.app/Contents/Resources
- find the file defaultdesktop.png
- rename it to defaultdesktop_backup.png
- copy the image that you want for mission control to the folder and name it defaultdesktop.png
- open terminal and type
now your mission control will have a new background