Symbolism, Imagery, Allegory
Court's in Session
One of the weirdest things about Ezra Mannon is that he's always there—even when he's not. Characters constantly talk about him, and his memory haunts Lavinia, Orin, Christine, and others after he's dead and buried. The portrait of Ezra that hangs in the study reinforces that idea.
But oh, man—whoever said a picture is worth a thousand words wasn't kidding. O'Neill says that, in the portrait (painted ten years ago), Mannon's face is "cold and emotionless, and has the same semblance of a life-like mask that we have already seen in the faces of his wife and daughter and Brant." It's that portrait that makes poor old Abner Small think he's seen a ghost in The Haunted—unless it really was a ghost he saw.
We also know that this is a portrait of Mannon in his judge's robes, which is definitely important in a play that has to do with crime, justice, and revenge. Lavinia wants to be a judge just like her daddy, and punish Christine and Brant for their crimes. And, when Orin's writing his version of the Mannon Family history, he looks up at the portrait and asks it a question: "The truth, the whole truth, and nothing but the truth! Is that what you're demanding, father?"—like he's a witness testifying under oath (The Haunted, Act 2).
The huge portrait hanging on the wall is supposed to suggest that Old Man Mannon's somehow always watching what's going on, making us always remember the power he has over the lives of others. When other characters are in the room with the portrait, you can imagine it hanging high above them, almost like Papa Mannon's looking down on them like a king sitting in his throne, always judging.