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