The Dock wallpaper is found at /System/Library/CoreServices/Dock.app/Contents/MacOS/Dock/
(to get to it in Finder choose Go>Go To Folder…
from the Finder menu or hit Cmd-Shift-G)
The file you want called [email protected]
You'll need an admin password to change anything in that folder, and I'd advise you to make a backup before fiddling with it. You need to log out and in again for it to have any effect, or force quit and restart the dock (from a terminal sudo kill Dock
will do that, then to restart it type /System/Library/CoreServices/Dock.app/Contents/MacOS/Dock
).
If someone has removed the default, here it is (if anyone from Apple is worried about intelectual property it's a lookalike that I created with gimp on a debian system so it's all totally GPLicious. Yeah, right):