How is imagery used in Hamlet?

Imagery is used in Hamlet to highlight the theme of madness. Following Hamlet's killing of Polonius, Gertrude declares her son to be as

Mad as the sea and wind when both contendWhich is the mightier. (act 4, scene 1)

This is a very powerful image, as it makes it seem as though Hamlet is an uncontrollable force of nature. That's certainly how he comes across to everyone at court.
The sorrow created by madness, this time Ophelia's, is also expressed through powerful imagery. In act 4, scene 5, for example, Horatio, after seeing how much of a state Ophelia is in, describes sorrow as coming like "battalions." A battalion is a large body of troops ready for battle, and by using this expression, Horatio conveys the sense that Ophelia's madness, on top of everything else, has caused such sorrow that its immense power cannot be held back.
It's as if the entire court has been attacked by an enormous deluge of sadness and has no defense. First, there was old King Hamlet's death and then the death of Polonius. Now, on top of everything else, Ophelia's rapidly losing her mind.