Images are also considered "fixed size data" in the textbook. I think this is for pedagogical reasons, the sort of simplifying-lie-that-is-cleared-up-later that is common in science education. In the first section, students are encouraged to think of strings and images as atomic data to be processed all at once using built-in functions, but in the "arbitrarily large data" section, the authors give you tools for converting images and strings to lists (of characters and pixel values).

(Note: since Racket supports exact bignums, in a sense numbers are not atomic either! Only Booleans ;-)