It sounds like your presentation could be about their friendship. You could present pictures of all the things they did as friends, such as jumping out of the tree into the Devon river, doing chin lifts, swimming in the ocean, sleeping on the beach and training for the Olympics of 1944. Then, you could compare and contrast the two boys by showing the differences between their personalities. Gene would be the one studying and Phineas would be the athlete. Then, as the story shifts, you could show Gene as the athlete and Phineas on crutches. Sad, yes, but true to the story. Try to find quotes of things that each boy said that characterizes themselves or the other perfectly. For example, Phineas discovers that Gene needs to study and says, "I didn't kno you needed to study. . . I didn't think you ever did. I thought it just came to you" (58). This shows what Finny's view of Gene's personality was, but it also shows how much he looked up to his friend as well.
Also look to the links below that discuss the two boys' characters for more insight.