At my high school in Indiana, the senior portraits were a big deal. In the yearbook, the younger grades had the typical school picture - grey backdrop with just their face and shoulders. Each grade had a 2 page layout with just the photos and the students names.
For seniors, you had your picture taken by a professional photographer. Besides using all of the different backdrops in the studio, you could pay extra to have a photo shoot at the place of your choosing. In the yearbook, you had a decent sized picture, plus a listing of all the activities you had been involved in while in school.
People also picked out two or three different poses and had a bunch of wallet sized photos taken. You exchanged photos with all of your friends, and also included them in your graduation announcements.