To change the Lego-inspired background in Dashboard (you'll need an Administrator password to do this)
navigate your way to/System/Library/CoreServices/Dock.app/Contents/Resources/
find the image "pirelli.png" and rename it to "pirelli copy.png"
find an image you want to use (something that will tile!) and copy it to the Resources folder
name it "pirelli.png"
The next time you restart/log in, your background will have changed to the new Pirelli. Alternately, you can open System Preferences and Mission Control, then deselect and reselect the option to show Dashboard as a Space.