Memento Photos are one of the game’s collectibles and don’t really have a big importance on anything in the game. All of them, except for one, can be found by completing the Extract the Wandering Mother Base Soldiers Side Ops, with each one rewarding a photo. If you head to the Medical Platform at Mother Base, you will be able to find Paz there in a room.
You can show her these photos, but only one at a time within a certain time period. To trick the game, simply exit her room and use your Phantom Cigar to quickly pass time, enabling you to show her another photo. Once you’ve shown all ten of them, you can head outside of her room and right on the wall will be the final photo, earning you the Reminiscence trophy/achievement . Note that you can show her this photo for one final cutscene.
Note: After showing Paz some of the photos, there will be a cutscene and she might not wake up to greet you like before. Just continue playing some missions or Side Ops, then come back and she’ll be fine and allow you to start showing her more photos.