There are 11 memento photos to obtain. 10 of these are obtained from completing the side-ops "Extract Wandering Mother Base Soldier". Go to the second floor of Mother Base's Medical Platforms main deck. Look around and you will find an area you can go inside, and a scene will trigger. Bring the photos to this location (best done each time you extract a wanderer, as you can only bring one per visit), and once all 10 have been brought, you will find the 11th photo here.