Symbolism, Imagery, Allegory
You'd think the moon had a stinkin' selfie stick given how many photos we get of it in the story. Photos are a big deal to both TJ and Jamie. It starts out as a hobby that TJ does to make sightseeing more interesting, but pretty soon it turns into a way for the siblings to communicate with one another. During TJ's absence, he sends his sister rolls of film, which she develops for him. It's a way for him to show her what he wants to share about his experience and for them to stay close despite the miles between them.
Jamie explains that "each little piece of the picture is like part of a puzzle, and the more defined you make everything, the more your picture tells a story" (7.21). We love the idea of a picture being a puzzle (so long as it's not one of those with a billion tiny pieces). Only once the picture gets developed can it be something complete. By developing his pictures, Jamie gets a glimpse of what it's like over in Vietnam through TJ's eyes, bringing her close to her brother rom afar.