A string is a second form of symbolic data. In contrast to symbols, strings are not atomic. They are compound data, which we discuss later in the book. Otherwise we ignore strings, and when we use them, we act as if they were symbols. Like symbols, images don't have any a priori meaning, but we tend to connect them easily with the intended information.
Images are values like numbers and booleans.
They can therefore be used inside of expressions. Most often though, we give images names because they are typically used by several functions.