What is a pin up?

[saber]

I’ve been wondering because I always see them in the event page

[amy]

They're pictures that you can get. These aren't the ones you get by increasing girls' relationship levels, you get them mostly/completely through events.

You can see the ones you have (if any) by going into the More tab, then the Memory Album, then clicking "next" until you're past all the girls you've unlocked.