Heart of Darkness: Metaphor Analysis

The Congo River
Literally, the Congo River is the means by which the Europeans enter the region.  It is also their main method of transportation.  Marlow equates the river with a coiled snake.  Thus, it is a symbol of danger; it lies in wait, ready to strike.  It should be noted that Marlow's journey upriver, into the heart of the Congo, is a very time-consuming and arduous.  This suggests that the journey into oneself is both a slow and difficult task. 
Darkness
Darkness is an important symbol in the novel.  Marlow's tale opens and closes in darkness, and many of the novel's significant events happen in darkness as well.  As a symbol darkness represents the core of our being.  It is what's left after we throw off the illusions and comforts of civilization, a tremendous emptiness.  It is the symbolic equivalent of Kurt's message "The horror!  The horror!"
Decapitated Heads
When Marlow first arrives at Kurtz's station, he thinks he sees a line of ornamented posts surrounding the main house.  Upon closer inspection he realizes that the posts are topped by human heads.  These heads provide the most direct evidence of Kurtz's "unsound" methods or, to some, his insanity.  They also symbolize the savage and brutal nature that lies within each of us.
Ivory
Ivory is the main reason the Europeans come to the Congo.  It is a symbol of lust and greed.  As an object, ivory is something pure and beautiful, but the means of acquiring it are very ugly.  Ivory also operates as a symbol of whiteness or European culture.  
The Intended
Literally, the Intended is Kurtz's fiancee.  As a symbol she represents all of the values and morals that Europeans "intended" to bring to the Congo.  Of course, their lust for ivory makes any altruistic actions impossible.
 
Kurtz's painting
Kurtz's painting, an image of a blindfolded woman carrying a torch, is a significant symbol in the story.  Readers may be reminded of the blindfolded symbol of justice.  In this way, the painting can allude to the tremendous injustices that take place at the hands of the Europeans.  But the figure also represents European colonization.  The torch is the "light" of culture that Europeans are supposedly bringing to the region, while the blindfold represents the "blind" eye they must turn to accomplish their activities.