[ FAQ Question #261: Loyalty Userpics ]
Brought to attention by jeffreyatw, thanks!
Put simply, you get one free userpic for every three-month segment in the continuous—thus “loyalty”—paid time of your account as of March 15, 2006.
I'm up from 15 to 38; there's no way I'll ever use 'em up anytime soon! ♥♥♥