they were to be be kept as souvenirs
Souvenirs of what ? If Lee took the photos of Walker's house ( And I believe that he did) Why would they be "souvenirs"? I suspect that Lee took them as "evidence" that the owner of the vehicles was visiting walker......
This is a genuine mystery that has never been explained. Oswald burned some of his papers regarding Walker, but kept the photos taken from the back of Walkers house. If those photos were ever found, Oswald was leaving himself wide open to being charged with attempted murder which i'd image would have given him a nice 20 year term or more in jail.