I don't know that you could pick out one image and say that it is the most significant image in Chronicle of a Death Foretold, but one important image that is central to the novel is Nasur's death scene. 

The repeated stabbing of the victim, in fact, the tremendously excessive stabbing of the victim, plays against the idea that this was an honor killing, and turns the killing into a revenge killing.  The scene also plays against the neglect of the townspeople in not warning Nasur of the plot on his life.  The images created in the scene also play against the decision of the court and public opinion that this was an honor killing. 

Whatever faults Nasur may or may not have had, the brutality of his murder adds to the depth of the questions concerning tradition, morbidity, mob mentality, and honor inherent in the novel.