Memory
Photos, faded and bent, are physical memories. While neither they nor memory can be fully objective (and therefore remember exactly as it happened), photos act as extra limbs, prosthetic memories. Before photography, memory was the only way of "remembering" a person, event, etc... Now that photography is well in place, it not only supplements (supplement: supple elements which interact and affect) but mediates and even replaces memory. Events are no longer one time, but all-time (video/film) or all-past (still photos) to be manipulated in our (malleable) memories.
To remember is always a selective act: one cannot remember everything about an event, person, etc... In fact, no one can remember everything. Remembering requires that we store information, some of which displaces other things (and which may eventually be displaced itself). Forgetting is inherent in remembering; it begets memory. The photos that Leon and Rachael have are (un)safeguards against this process. For Leon to have memories, he must get his photos. If Rachael is to survive the fracturing of her identity, so must forget her memories, throw out her photos, and begin again.