Although the first few chapters of Mark Twain's novel, The Adventures of Tom Sawyer, deal with humorous episodes that introduce the principal characters, the main conflict of the story picks up later on and deals with Tom's "life-threatening entanglement" with Injun Joe. Tom and Huck's witnessing of the murder of Dr. Robinson eventually comes out at the murder trial of Muff Potter, and Injun Joe escapes. The boys' worries about Injun Joe seeking revenge upon them become a focal point for most of the rest of the story, and the various treasure hunts and adventures in the cave all revolve around Injun Joe in some way. If I were creating a poster, I would probably focus on all or part of these characters--Tom, Huck, Becky Thatcher and Injun Joe--in the cave.
I think you could look at the main conflict in two ways.
First, you could look at what the boys see as the main conflict. If you look at it this way, the main conflict is between the boys and Injun Joe. They spend much of the book worrying about Injun Joe taking his revenge on them.
A second way to look at it is more philosophical. During the entire story, we see Tom in conflict with the rules of the adult world. We see him cheat to win the Bible, cheat to get out of whitewashing the fence, etc.
One of the main conflicts in the novel is Tom's desire to follow his own desires and fantasies at the same time that he wants to win admiration and acceptance from the adult world and from Becky Thatcher. At the beginning of the novel, Tom devotes himself to escaping from responsibility and to using his cleverness to trick others (for example, when he convinces other boys to whitewash the fence for him). Later, he has the chance to prove himself a capable young man by helping to rescue Becky from a cave in which they are trapped and by finding stolen gold. He eventually proves himself able to win respect from the adult world and from Becky. The poster you make might portray Tom as divided between the world of adventures that he wants to follow with Huck (for example, hiding out on an island) and the real world in which he must rescue Becky, find a killer (Injun Joe), and discover the gold Injun Joe had stolen.