"The LORD God said to the serpent... I will put enmity between you and the woman, and between your seed and her seed; he shall bruise your head, and you shall bruise his heel." - Genesis 3:15.
Eve, the first woman, was a virgin at the time that she was tempted by the serpent in the garden. Thus, Eve, a virgin, conceived disobedience and death, whereas, Mary, a virgin, conceived the Word in obedience and brought forth Life. So, she is shown here carrying the Word Incarnate, Jesus Christ, who is destroying the serpent of sin and death.
This painting is in one of the side chapels of Salamanca Cathedral.