A third way to think about this essay is in terms of theme.  Your teacher might have meant:  what is the thematic significance of imagery in the play.  To answer this, you need to figure out two things:  first, listing the images, figure out what they mean and what senses they appeal to; second, figure out the theme of the play:  what is the author trying to say?  why is the universal statement about life that she/she suggest?  Images about fading flowers, for example, might help develop a theme about the brevity of love or life, that both must in the long run end.

Images also might add up to create a mood (perhaps one of foreboding, such as in Poe), or a tone (perhaps satircal, such as in Animal Farm), and mood and tone contribute to meaning.  A selected group of images might also serve to characterize a character in the play.

Your teacher probably wants you to give specific examples of the imagery used in the play and then tell how the use of this imagery adds to the play. Why does the playwright use the imagery? Imagery appeals to our five senses: taste, touch, smell, sight, hearing. What does the playwright use in the play to evoke an image in our head that lets us know what something in the play looks like, tastes like, smells like, feels like, or sounds like? That's what is meant by imagery.

For example, there are many references to decay in the lives of the Hunsdorfers that represent the theme of death in the play. The setting is the front room of the Hunsdorfer home, a rundown, wooden building that used to be a vegetable shop. The clutter of the house represents the broken bits and pieces of Beatrice's dreams. Just as she's trapped in this room during the whole play, she's trapped in these circumstances of her life. Think about the marigolds--what do they represent? Only through her marigolds does Tillie experience any growth and have the courage to pursue her dreams.

Once you get your examples of imagery that are used in the play, you can then tell how these examples make the play better or how they add to the themes of the play. I hope this helps. For more information, go to the links below. Good luck! 

When you write on the significance of imagery, you are talking about the influences your five senses have on the play.  How do you see things through your sense of sight, touch, taste, smell, hearing... It is not enough for you as the writer to just point out, "In this scene there is great visual imagery..."  What your teacher wants from you is to know the next step... the "SO WHAT?"

The use of fire adds to the child's sense of fear..

The smell of peaches brings back memories of ...