We've just implemented the concept of "inactive userpics". It's the beginning of something that people have been asking about for a long time -- "userpic add-on packages". Basically, in the future, people will be able to pay a small fee and get more userpics. In order to do this, though, we have to support "expiring" the userpics once account status changes.
If you've had a paid account in the past, and your paid account expired, the server will now flag your least-used userpics as inactive. They'll still show on old posts and comments, but you won't be able to use those pics again in new posts or comments. If you want to use those pics again, you can go back to a paid account, and you can start using those pics again.
In the past, you could keep using the pics until you wanted to upload a new one, because we didn't have a way to force you to remove the pics. This is a compromise between making people get rid of the pics entirely, and letting people pay once and keep using all their pics.
So... sorry :(