In the past two decades or so, there have come to be more and more pictures included in textbooks from all grade levels and in all subjects (at least in the United States).  This has happened for at least two reasons:

• From the pedagogical point of view, textbooks now include so many pictures because that is supposed to make them seem more interesting and less intimidating.  It is said that books with lots of pictures and other illustrations don't seem like as much work to read (not so many words on each page, not so many big blocks of text with nothing to break them up).
• From an economic point of view, the textbook publishers welcome the use of more pictures because it gives them the ability to charge more for their books and make more profit.

So, pictures are used more and more in textbooks because of both educational and economic reasons.