"Memento" means, roughly, "an object that functions as a reminder." Reminders are important in Memento: Leonard basically creates a pseudo memory system (through mementos) in lieu of the real memory he doesn't have. Leonard's tattoos are his most important mementos—he has all kinds of things written on his body so that there's no way people (aside from himself) can manipulate them, and there's no way he can lose them (aside from, say, dismemberment).

The photographs help with his recognition. They tell him the people he knows, the places he lives, the car he drives, the people he's killed or hurt. The difference between normal mementos and Leonard's is that these pictures are his world. Leonard's mementos aren't technically mementos; they can't be. Leonard can't remember the things he's taken pictures of or got tattoos of— he's actually learning something new from these things over and over again.

But "memento" doesn't just mean "something that reminds." It can also mean "something that warns." Many of Leonard's mementos have notes attached to them, warning him from being too trusting. Even his tattoos have warnings like "DON'T TRUST" and "NEVER ANSWER THE PHONE." Leonard is surprisingly easy going and nonchalant for someone without short-term memory, and he needs all of these mementos in order to protect himself from people who might want to take advantage of him. (Come to think of it, Leonard could use a picture of himself.)

Lastly, Memento (not the movie but a Roman Catholic prayer) is used at services to commemorate the dead. Memento (the movie) is sort of one long commemoration of Teddy. The film is also driven by Leonard seeking vengeance for his dead wife. It's also a kind of commemoration of Sammy Jankis, and possibly even Leonard himself.