Former soldiers of the Militaires Sans Frontières carried memento photos from the old Mother Base after it was destroyed in 1975. Some of those soldiers were recruited into Diamond Dogs by Venom Snake in 1984. Their mementoes were perceived by Venom Snake to be passed on to an amnesiac Paz Ortega Andrade.
Behind the scenes
The Memento Photos are key items in Metal Gear Solid V: The Phantom Pain. They are rewarded after completing each of the Wandering Mother Base Soldiers side ops (except for the 11th one, which requires that the player give all 10 in a certain segment of the game and then find it on the wall), and are integral to the completion of a side story involving Paz, or rather, a phantom of her.
Similar photographs can also be found on The Boss AI either at the Afghanistan Central Base Camp (prior to the completion of the related Side Op) or at Mother Base (after the completion of the aforementioned related Side Op), although in this particular case, they aren't actually collectible items.