Let us remember that imagery is any kind of description that builds a strongly visual picture of what it is trying to describe through the use of as many of the five senses as possible to convey the scene. Consider the following description of the girls as they begin to play in the Professor's yard and wonder if they are being watched:
With one accord the girls moved warily towards the window. Closer and closer until their noses were only inches from the dirty panes. Then Melanie breathed a sigh of relief. "There's something like a heavy curtain hanging clear across it. He couldn't see through that."
Note the way that we have a clear visual image of the "dirty panes" and the "heavy curtain" that Melanie sees covering the window. In addition, we have the "sigh of relief" that Melanie breathes which of course appeals to our sense of hearing. We are able to imagine the way in which the half-terrified, half-excited girls creep towards the window and then their relief as they see that nobody could actually be watching them.
This is just one example, and of course the novel contains many different examples of imagery. As you read, pay particular attention to images that incorporate different senses and key words that help describe the scene.