The authorities (Poskim) discuss the law of a book that has letters written on its edges. Regarding this, the Rema was lenient, while the Magen Avraham was stringent.

However, in the case of a specific picture or illustration, the matter is forbidden (as it may constitute the prohibition of Kotev—Writing/Creating an image—when the book is closed). This restriction applies unless the illustration is a generic or decorative design, as is commonly found on many books.