From what we've been able to gather ("we" being random support volunteers) is that the current default userpic plus the next two most used userpics (by keyword, over the past 60 days) are made active, while others are inactive. Entries made using the default picture and no keyword are not counted. We're unsure if posts to communities, or comments, count for this purpose.
Anyhow, this is a source of a good deal of support requests, and it'd be nice to have a FAQ that pointed out exactly how "most used" user pics are calcualted.
