Age
The narrator—all the members of Lockwood and Co. are young. The description of age differential uses imagery to pound home the point. "I don’t know how old the man was—when I see someone older than thirty, time sort of concertinas for me—but he was definitely closer to coffin than crib." This is a subtly effective use of imagery with the comparison of perceptual difficulties being manifest as the concertina's ability to be either very long or very short. That life beyond her point in the process is either birth or death offers insight into the psyche of the narrator, Lucy.
The Smell Switch
For just a short period of time, Lucy finds her sensory abilities incapacitated. It's too dark to see, no sounds to hear, and even her ability to smell has been dampened. "Then, like a switch had been touched, my sense of smell suddenly flicked on. I got mold and dirt and the bitter tang of blood, all at once, as if someone had shoved it all violently up my nose." The use of imagery here lets the reader still share her blindness in the pitch-black darkness. The comforting familiarity of recognized odors is given a twist of unfamiliarity with the final image in the description.
Holly
Lucy has been the only girl, but now there is Holly. "So she was well-proportioned. So her hair was all glossy. So she looked as if her lips had never been the wrong side of a second doughnut in her life...if I’d thought hard enough about it, I could have found something flawed about the width of her thighs." The magic of this use of imagery is that while it paints a picture of Holly's physical attributes, it does so through the subjective perception of Lucy. And in doing so, the imagery can also be used to get a glimpse into Lucy's self-image.
Echoes
Ghost hunting is all about stirring up the echoes of the past. "It was like coming to a valley where someone had once shouted, loud and joyously, and the echo of that shout had resounded between the hills and lasted a long time." Lucy is using imagery to facilitate the concept of ghostliness. An echo is a perfectly constructed image of a ghost, which is itself merely an echo of a person now dead.
