What is interesting about this story is that there isn't any "real" conflict in the story. The man who kills the old man even explains that he didn't have any real motive to kill the old man except that he wanted to, especially after the eyes stared at him. It is truly senseless. To call this a "man vs. man" conflict would be accurate because one man killed the other, but it isn't very satisfying.
As for the the imagery of the story -- it is abundant. Remember that imagery can appeal to any of the five senses, so the most important imagery of the story is aural imagery -- sound imagery. The man kills the man and then dismembers the body and buries it under the floor boards of his house. There is some visual imagery in that, but the point of the story comes from the sounds. The man, as he is being questioned by the police on an unrelated matter, imagines that he hears the still-beating heart of the man he killed. At first it is almost impreceptable, but as his guilt builds, so does the volume of the heartbeats. By the end of the story, the volume is overwhelming and the man is convinced that everyone can hear it. In this moment he blurts out the truth of his actions.