For one thing, Richard Cory seems godlike as the "people on the pavement" look up to him and "wish [they] were in his place."  So, actually Richard Cory becomes a symbol to the ordinary people, does he not? (A symbol often means more than itself as its literal meaning suggests more than what it is.)  Perhaps, then, with Cory at the center, you can depict around him all that he suggests--wealth, gentility, luxurious idleness, refinement, royality. (Find pictures of objects that can symbolize these abstract nouns.)

While Cory represents all that is enviable in a golden light, the ordinary people go "without the meat, and cursed the bread" in envy and a type of darkness until Richard Cory descends into the ultimate darkness of death

This could be a symbol poster.  Use your imagination and have fun with this; think of an object that can represent darkness, refinement, royality, etc.

Here's what I would suggest for a poster.  Have a poster that has two layers, the top layer being able to open up, like a set of double-doors.  So, you would get two posterboards, and cut one in half, and glue the right and left edges on top of the first one so that it opens up like a door.  Then, on the top layer, put a collage of very beautiful people.  Find models, male and female, and glittery beautiful things.  That way, when we look at the poster, on the surface layer, it is covered with pictures of people and images that we all envy because they seem beautiful, successful, happy and perfect.  But then, when you open the flaps, put ugly, unhappy, dark pictures.  Make a collage with words that say "unhappiness", "don't judge a book by its cover", and pleas of help.  So, when you go under the surface of the beautiful people, to what is inside of them, you realize that they might be miserable.  Does that makes sense?  I hope so.

This all can represent the theme of the poem-how surface appearances are deceptive.  The poetic devices in the poem are rhyming, metaphors ("waited for the light" a metaphor for a relief from hard, dreary, every-day work), imagery-using the five sense to describe something ("he fluttered pulses" and "glittered when he walked").  You could write those phrases on the poster somewhere, if you want to represent them.

I hope that idea helps!  Good luck with this assignment.