Memento Photos is a type of collectibles that can be found in Metal Gear Solid V. There are eleven photos to obtain and for gathering all of them you receive the Reminiscence Achievement/Trophy.
Ten out of eleven photos can be obtained by completing Side-Ops from the Extract the Wandering Mother Base Soldiers type. These are missions 51-60 (Extract the Wandering Mother Base Soldiers 01-10). The walkthrough to this mission can be found in the chapter about Side-Ops. Every mission of this type is similar, because your task is to locate a lost ally (the best way is to use D-Dog's help) and extract him. The difficult part is that the soldiers start running away when they see you, so you need to sneak up to them and knock them out. A sniper rifle will be very helpful when completing this type of missions. When knocking out a soldier from distance, you don't have to worry that you will scare him away.
Obtaining the last, eleventh photo is more troublesome. After collecting at least one photo by completing a mission from the Extract the Wandering Mother Base Soldiers type, go back to Mother Base. Choose Medical Platform as your landing site. Take the stairs to reach higher levels and locate the entrance shown in the pictures above (there is a blue light above it). When you're in the corridor, locate the door leading to the medical room, where Paz is.
After watching a cut-scene, you can give the first photo to Paz. The girl will go to sleep, which means that you cannot give all the remaining photos to her right now. You can solve this problem using Phantom Cigar to fast-forward time or by leaving the Mother Base and going back.
Give all the photos to Paz so that she can watch all ten of them. Then, you just need to leave the room and look around the corridor to find the eleventh Memento Photo. After collecting it, you will unlock an achievement.