I don't know where they're stored, Byron - it is obviously deep in the system somewhere because I asked that question once and never got a definitive answer either. If they're deep in the system, I wouldn't recommend messing around with that - you might mess up more than what you're trying to change.
In Lion, location of the user accounts images is:
Macintosh HD > Library > User Pictures
Hope this helps (despite the belated reply).