For an assignment for English, what visual representation can I make for the story of Macbeth? The visual representation needs to be of a symbol that is not obvious in the story. This is supposed to be a visual essay with a thesis.

In terms of an "obvious" symbol, I imagine you are being advised to avoid using the dagger Macbeth sees before him or superficial representations such as castles or Highland landscapes, which might represent the setting but do not represent the core of the story. Personally, I would argue that the best symbol you could use to represent Macbeth visually would be blood.
Blood recurs throughout the story as a motif, and it has multiple meanings. Blood as a representation of guilt—the "blood on my hands" concept—pervades the story, both before and after the actual murder is completed. When Macbeth is mustering the courage to kill Duncan, he imagines a dagger before him that represents the instrument with which he will commit the murder, but, more importantly, as he contemplates the deed, he begins to see "on thy blade and dudgeon gouts of blood." Macbeth determines that although the dagger is not real, "it is the bloody business" of the murder that is making the dagger appear to him. Blood is a symbol offered to Macbeth within the play by his own "heat-oppressed brain," and it symbolizes guilt.
Later, when he has killed Duncan, Macbeth questions whether "all great Neptune's ocean" could "wash this blood / Clean from my hand?" He feels sure that the evidence of his guilt, as represented by the blood, is so obvious that it will indeed stain all it touches: if he were to touch the sea, it would turn red.
At first, Lady Macbeth tells him he is being ridiculous, saying that although her hands, too, are "of your colour," her heart is not "white" as Macbeth's heart is. Later in the play, however, we see the symbol of blood rise again; when Lady Macbeth succumbs to her guilt and begins to go mad, she muses while sleepwalking, "who would have thought the old man to have had so much blood in him?" The blood here seems to represent not only Duncan's literal blood, but the following question: who would have thought that murdering him would create such an endless ocean of guilt? Lady Macbeth echoes her husband's earlier plaint when she says, "will these hands ne'er be clean?"
At the end of the play, when Macduff and Macbeth are about to fight to the death, Macduff rightly calls Macbeth "thou bloodier villain / Than terms can give thee out!" It is as if he is recognizing the extent to which Macbeth's guilt has been painted upon him in the form of blood, both literally and metaphorically.