The blemish
The blemish is a literal depiction of a character flaw in the Marquesa. By making the flaw obvious, but concealing her identity, she makes us see that the blemish represents her unsavory ability to get what she wants on her own terms. She's deceitful.
The nude portrait
The depiction of the woman's identity in paint is a way of indicating certain qualities of the murderer. For one thing, she is turned with her back, but she's standing with a mirror that shows her naked body. This is an indication of the woman's ability to seduce, to be coquettish and demure. Her scrubbed out face shows the toll that her shape-shifting deception has taken on her sense of self. Not only do they not know who she is, but she doesn't even know who she is when she looks in the mirror. The final stroke of beauty in the painting was the murderer's own blade in the back of the painter, because after all, her greatest character trait is that she will steal power through whatever means she can just to protect herself.
Rich, high-class imagery
The novel takes place in a sophisticated, upper-class environment, because the Marques and Marquesa are highly significant members of their community. So the details of their home, the object symbols in the book and even the plot's central image, the painting itself, are all images of extravagant wealth and luxury, adding a lovely aesthetic to the story.
The imagery of violent murder.
Instead of merely reporting the murder to the reader, the novel depicts the scene in full graphic detail. The violence of the image is passionate and lovely, a painter slain who uses his last moments to paint the missing detail on the body—and yet it is gruesome. This duality complements the duality of the murderess and her ability to garner praise and still be evil enough to murder in cold blood.
The imagery of attraction and seduction
In the portrait, the faceless woman is twisting flowers into her hair, adding something beautiful to herself by pulling it into her braids. This seductive imagery is an indication of the character's ability to draw things toward herself, to attract and seduce. The Marquesa is said to be undeniably beautiful, and the doctor compliments her on her ability to bewitch him without even resorting to her physical beauty. These are all instances of attraction imagery.
