If you have not taken your senior picture, you just can get your ID picture the same way you have always obtained it. Even if you never take your senior picture, you will get a photo on your ID.
Not every student is enrolled at the start of the year. Some of these students are transferees. That is why there is always a retake session later on in the summer or start of senior year. There are many people who do not get their senior pictures during the summer for a variety of reasons (like they forgot, they were on vacation, or they did not like their picture, etc).