IIRC (my memory is a bit fuzzy on this, as I've never had a paid account, so this never applied to me), if you allowed a paid account to revert to free status, any userpics you had uploaded remained. However, FAQ #193 indicates that any usepics over what your account is currently authorized are inactivated. The userpics most used in journal entries over the past six months (use in comments is not a factor) are the ones that remain active. Finally, inactive userpics still show up in the posts in which they were used, at least for six months (inactive userpics are removed from the server at the six-month mark).

Note that for a Free account, there's a limit of six active userpics (Free but ad-plagued Sponsored + accounts can keep fifteen active userpics).