What was the thing that Lieutenant Jimmy Cross carried?

From the text:

First Lieutenant Jimmy Cross carried letters from a girl named Martha, a junior at Mount Sebastian College in New Jersey. They were not love letters, but Lieutenant Cross was hoping, so he kept them folded in plastic at the bottom of his rucksack.

In his wallet, Lieutenant Cross carried two photographs of Martha. The first was a Kodacolor snapshot signed Love, though he knew better. She stood against a brick wall. Her eyes were gray and neutral, her lips slightly open as she stared straighton at the camera.