The Adventures of Tom Sawyer by Mark Twain is full of little conflicts. My personal favorite is the conflict in the beginning of the story when Tom is attempting to find a way out of painting the fence. It's a great scene because not only does Tom not paint the fence, but he convinces the other kids to pay him for the privilege of painting the fence.
The main conflict in the book though is centered on Tom and Injun Joe. Tom witnessed the murder of Dr. Robinson. Injun Joe escapes, and Tom is terrified that he will come back in order to seek out some kind of revenge for Tom's part in the trial.
For your poster, I would focus on the Injun Joe conflict. Use a lot of adjectives to describe what type of person Injun Joe is in order to highlight exactly why the conflict is so important and why Tom is so afraid of Injun Joe.