I figured it out.  They're stored as <user id #>.ext in the avatars directory.  If the file exists for the user #, they get an avatar.  If not, no avatar is displayed.

I'm just wondering where users' avatar choices are stored.  I looked around in the DB and couldn't find them, so I figured I'd ask here.

BTW, I'm really happy with PunBB so far, and I'm having fun integrating it into my site.  It's nice to have a lightweight solution that's so customizable.  I used to use miniBB, but I'm much happier with Pun.  smile