I created an example to demonstrate my expectations for the assignment. My example appears below (with a full year of wear and tear).
By including a photograph on my own notebook, a lot of students followed this lead and included pictures of family and friends. It may seem like an expendable assignment for high school students, but I definitely plan to do the same next year. The notebook covers gave insight into my students' personal lives, and allowed me to learn even more about them as people outside the classroom.