The margins designate the safe zone for placing important elements like characters and text. They are NOT where the page will get cut during printing so feel free to stretch the background and other pictures beyond them, right to the page edges. Just make sure all the text (and other important elements) stay inside to prevent them from getting cut off during printing. Oh, and if you’re planning to sell your book on Amazon, then it’s REALLY important to keep all the text inside. Just so you know!