How does the author use imagery in the novel, The Night the Heads Came?

Example of Imagery:

The creatures are tall, taller than Tim and me, with two long, pale arms that seem boneless, like tentacles. I can tell they are very thin, even though they are wearing loose, sleeveless robes that hang from their long necks to the floor. The truly horrible thing about them is their heads, simply because they are so tiny in relation to their height, about the size of tennis balls. The heads are smooth and gray and almost featureless, with one lidless eye in the front and another in the back, and underneath the eyes a mouth like a line without lips that seems to go all the way around the head.

In fact, looking at their heads and the way they move, it becomes apparent that these creatures don't have a front or a back; they are the same on both sides, gliding backward or forward without having to turn around.