Audience
The actors in the play break the fourth wall throughout the plot. The imagery created relates to how they need to relate to people in order for them to understand their point of view, and reveals the neediness of each character to be heard from this point of view.
Dark Room
Penny tells Ronnie that Douglas has come on to her sexually in the dark room. The imagery of this young girl in a room, alone with a man who is meant to be a father figure coming onto her, becomes the terrifying image of a man with horrible secrets that he stores away in the form of a "hobby" shooting photographs.
Bright Lights
The lights at the end of the play "bounce up bright and full". This imagery creates an ending that suits the style of the play, which is realistic but set in a way that fluctuates between present, past and future. The lights are a metaphor for how this is a defining moment for each character's destiny.
Mt. McGinty
Mt. McGinty is on fire in Act III. The imagery evokes the connection between how, like the land, Douglas and Ronnie's home is also dry and barren; and at any moment could catch fire.