Yes the missing user icons is unexplained and nothing I've tried has resolved it.
It seems to have occurred as part of the 10.8.2 update for me.
No user shows any of the default or custom user icons available in the users system preference. All the Apple icons are present in the /Library/User Icons folder as well as custom icons but they do not appear within the system's panel period. No icons appear there. You are able to add icons to it -per user. Any done in this manner are saved in that user's preferences so this does not affect the system level. I have tried removing preferences. I have tried removing the icons and adding them back including as root but all changes are applied to the user only. I have run Diskwarrior, repaired permissions, etc. And nothing changes this.
I cloned the image to another laptop, deleted all preferences from main library, restarted, logged in expecting it to be a preference issue. Still no user pictures shown for default user. In case the original users were corrupt, I created a new user and logged in with it. It was given one of the OSX pictures randomly chosen by the system. I logged in as user and still no pictures are shown. Tried to do a reinstall of 10.8.2 combo update and system refuses saying I cannot be done. Booted from recovery partition and it allowed an internet recovery.
No change. Took a brand new laptop and used user migration to copy the users over to it with all settings. Broke that system as well. Repeated creating new users, etc and no change. This is really annoying. I can repeat migrating users only and then partial settings to try to narrow it down but its time consuming.