I'll try to track them down for you, but you should back them up first in case it causes some problems, or in case you ever want to revert.
EDIT: Found them.
Go to /System/Library/CoreServices. Right- (or control-) click "Dock", and click "Show package contents". Then go to Contents/Resources. You'll see "trashempty.png" and "trashfull.png". BACK THEM UP, AND NOTE THEIR ORIGINAL LOCATION somewhere where you'll be able to find it later, and replace those files with the images you want, making SURE the names are correct, and the EXACT same as the originals.
Then restart. (Logging out and logging in may be sufficient, but it's probably safer to restart.)
Good luck!
Message was edited by: joshz