Loyalty Userpics (Thanks to camillabloom for the heads-up.)
Loyalty Userpics are extra userpics awarded to users as a reward for their paid account history with the site. You recieve one userpic for every three months of paid time you've bought for your account, and one userpic for every continuous 12 months of paid time your account has held. ... Permanent account holders will always have the maximum amount of Loyalty Userpics offered to date.
I don't think there's been an official announcement about this yet and I don't know whether it will be retroactive for paid accounts. But I have my extra slots, so I'm happy. :)
Now I just need some new icons. Whee!