In preparing for your assessment, you may be helped by considering some of the images of power that appear in The Chocolate War.
In the book, there are several visual representations of social or group power. When the Vigils call in a student to give him an assignment they stand in a large group with the student isolated on his own. This physical arrangement creates a literal image of the Vigils power, which stems from their numbers and their unity. This is an image of many standing against one (which is also a theme of the novel).
The poster that Jerry keeps in his locker can also be seen as an image of power because it is powerful for Jerry. The poster of a man walking alone on the beach with the caption reading "Do I dare disturb the universe?" is part of Jerry's inspiration to take a stand against the chocolate sale and against the Vigils. The image here is of one person quietly taking on something much larger and understanding the unlikeliness of success.
These examples present images of two kinds of power, social power and inspirational power, each of which have a role in The Chocolate War and in everyday life.
The text also presents images of the power of authority (relating to Brother Leon) and images of poetic power (like the image of the goal posts looking like crucifixes on the football field).
When writing about images like these, you can think about the narrative context in which the image appears (what's happening in the story), the relationship of the image to the themes of the book, and also your own intepretation of the image.