When you modify your profile in any way a Nostr event is created where all profile details are squashed together in a single JSON block. To make profile pic albums possible:

1. Relays should not delete old profile change events so that Nostr clients could parse the older profile pic urls from the old JSON blocks. This is pretty wasteful but would work.

OR

2. Profile pics should be its own NIP where the user can publish only the new set of urls without other profile details. This would work but it would also fragment profile handling making it more complex.