There are a number of images integral to Alvarez fictionalized account of the real life Mirabel sisters. The imagery is used to reinforce concepts of oppression, liberation, life and death, to name just a few.
First, of course, is the image of the butterfly. The sisters are known in the Dominican Republic as "Las Mariposas," the butterflies. A butterfly is beautiful but has a very short life span, just like Minerva, Maria Teresa, and Patria.
Another recurrent image is that of window, from which the imprisoned women view the world. It is symbolic of their captivity as a people, not just of the girls themselves.
The black towel that the girls use as a signal is an important image as it symbolizes both death and oppresion.
Yet another is the image of the "crown of thorns" referred to by both Patria and Dede. Patria's son is martyred to the "cause"; it is also the name of one of the plants that Dede grows in her garden, symbolizing her own regret and the cross of her decision that she must bear for the rest of her life.