After Adam and Eve sinned in the garden, the curse that was pronounced on them was followed by the blessed promise of the coming Savior in Gen. 3.15, the protoevangelion ("first gospel"):
"And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel."
So painters portraying the death of the Savior have often portrayed a serpent beneath the cross, crushed in the head. The beauty of Gen. 3.15 is that its theme keep recurring through the Old Testament, reminding us that through the sad days and times of our sin the Savior was coming, and that the Savior's work would be to destroy the serpent who holds us in the thrall of sin.
The first instance is when St. Jael assassinated the general Sisera. I've already pointed out (in this post) how Jael is like the Virgin Mary and how her actions foreshadow the work of the Savior. And here's another, similar, foreshadowing.
In 2 Samuel 20, Sheba, a man specifically said to be "a man of Belial" (a follower of the false Baal religion) leads a rebellion against God's anointed, King St. David. Sheba holes up in the city Abel, of Bethmaachah, and Joab besieges the city in an effort to squelch the rebellion. However the city is saved by the intervention of a "wise woman" (vs. 16) who pleads with Joab on behalf of the city, and exacts a promise from Joab to save the city if Sheba is delivered. The wise woman (who is likewise a picture of the church, as is the Virgin Mary) intervenes likewise with the men in the city, and Sheba's head is thrown over the wall.
Sheba here pictures Satan -- the adversary whose head is crushed by the Savior.