They are unlocked through her story.
This has actually been bugged since 13.2 ( I made a post in the Patch Note comments at the time); Autumn's Event folder in the PC stays a Locked folder icon after completing the event, so the pictures are inaccessible.
Honestly can't wait for v14, so excited to see the next part of Autumn's story in particular.
And the rest of the girls, in particular. They're all my favourite, I can't lie.