An image/likeness is a representative of a being, usually of a deity or a king. In functional terms, it stands in for the deity or king. Through it, the deity or king does his or her will, and the image exists in a place to express the domain of the one it represents to others.
When we discuss humanity as an image, we must keep this biblical and ancient Near Eastern view in mind. Image has nothing to do with ontological characteristics of the image, as an image can literally be any likeness and sometimes just an object with no likeness (e.g., an Asherah pole). An image has to do with its function. Who does it represent? What domain is it claiming for its deity or king? These questions have to do with its role.
This is why the image in the Bible has to do with joining God in His work as Creator. His creation is not complete until the world is full of human images who have overcome chaos. The first humans are made to begin to fulfill this role and other humans after them were meant to continue it.
So if humans rejected that role and decided to go with the one offered to them by the devil, what are they now but images, not of God, representatives not of His rule over their domain, but the devil's.
Hence, the Scripture tells us that the god of fallen humanity and its world is not YHWH, but the devil (2 Cor 4:4). We were in his domain, the kingdom of darkness, in his rule, representing his rule, until we were transferred from his kingdom to the kingdom of the Son (Col 1:13). Ephesians 2:1-2 states:
And you were dead in the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience”
The devil is the prince who rules from the invisible realm through the sons of disobedience. Hence, as he is a destroyer/murderer/liar, all mankind in him are described the same way (Rom 3:10-18). Those who are outside of Christ are his children (Matt 13:38; John 8:44; Acts 13:10; 1 John 3:10). They are federally linked to the devil as their father, and so they represent him in everything they do. That is what the image does, and that is why they are the image of the devil, and not of God.
Christ must restore us to the image or we are without God and without hope in the world. He is our only hope, the true image of the invisible God, the Son of God who makes us sons of God, and restores to us the representative role of the image that was lost when we rejected God's work for the adversary's.
Hence, Luther concluded that fallen man is Satan's image, not God's, and the Reformed confessions conclude likewise.
Large Emden Catechism (1551):
Q. 81. How should I understand this?
R. Indisputably, the image and likeness of God, in which man
was created in the beginning, along with all inclinations
for good, was lost in him.
Q. 82. How should I understand this?
R. This image of God was in Adam in the beginning, by virtue
of which he was immortal, holy, wise, and lord of the entire
world, and thus was endowed with the freedom and ability to
either completely execute or disregard the commandment of
God. However, the image of God in himself and in all of us
he so destroyed by his sin, that henceforth, all offerings
intended for goodness were utterly destroyed both in himself
and in all of us (
Reformed Confessions of the 16th and
17th Centuries in English Translation [Grand Rapids,
MI: Reformation Heritage Books, 2008], vol. 1, p. 607).
Scottish Confession (1560):
3. By which transgression, commonly called original sin, was
the image of God utterly defaced in man; and he and his
posterity of nature, became enemies of God, slaves to Satan,
and servants to sin.
Confession of the Spanish Congregation of London
(1560/61):
4:1. We confess that, man, at
his creation, having received from the hand of God the
powers of wisdom and the ability and will to know, love, and
serve his Creator, persisting in his obedience (which is
commonly called free will), received also a law (Gen. 2), in
the obedience of which he exercised these admirable gifts;
which, breaking by his own free will (Gen. [3]), at the same time was
marred from the image of God, and all the benefits that make him like God.
And from the state of being wise, good, just, truthful,
merciful, and holy he was rendered ignorant, evil, impious,
a liar, and cruel, clothed in the image and likeness of the devil toward
whom he moved as he departed from God, with the loss of that
holy liberty with which he was created (Eccl. 7; 2 Peter 2),
and thus was made a slave and servant of sin and of the
devil (
Reformed Confessions of the 16th and 17th Centuries in English
Translation [Grand Rapids, MI: Reformation Heritage Books, 2010], vol. 2, p. 376).
Belgic
Confession (1561):
14. We believe that God created man out of
the dust of the earth, and made and formed him after his own
image and likeness, good, righteous, and holy, capable in
all things to will, agreeably to the will of God. But being
in honour, he understood it not, neither knew his
excellency, but willfully subjected himself to sin, and
consequently to death, and the curse, giving ear to the
words of the devil. For the commandment of life, which he
had received, he transgressed; and by sin separated himself
from God, who was his true life, having corrupted his whole
nature; whereby he made himself liable to corporal and
spiritual death. And being thus become wicked, perverse, and
corrupt in all his ways, he hath lost all his excellent
gifts, which he had received from God, and only retained a
few remains thereof, which, however, are sufficient to leave
man without excuse; for all the light which is in us is
changed into darkness, as the Scriptures teach us, saying:
The light shineth in darkness, and the darkness
comprehendeth it not: where St. John calleth men darkness
...
Documents of the Debrecen Synod (1567):
First, since
the image of God was lost by Adam, it was restored through
the image of the infinite God, consubstantial and equal with
the Father, i.e., Christ was made to us righteousness, life,
truth, and sanctification; that is, He restored our lost
virtues (1 Cor. 1; Col. 1-2; Eph. 1, 3; 1 Cor. 15). “Day by
day, we are renewed more and more to His image through the
Spirit of God” (cf. 2 Cor. 3:18). “Put on the new man, who
has been created in accordance with God” (Eph. 4:24) ...
Therefore Christ, by the power of His deity, has restored
the image of God, the lost virtues (
Reformed
Confessions of the 16th and 17th Centuries in English
Translation [Grand Rapids, MI:
Reformation Heritage Books, 2012], vol. 3, pp. 17-18).
Craig's Catechism (1581):
Q. In whose image made He them? (Gen. 1:26)
A. In His own image.
Q. What is the image of God? (Eph. 4:24)
A. Perfect uprightness in body and soul.
...
Q. What was the craft of Satan here?
A. He persuaded them that good was evil and evil was good.
Q. How could they be persuaded, having the image of God?
A. They had the image, but not the gift of constancy.
Q. What things did they lose through their fall? (Gen. 3:17)
A. The favor and image of God, with the use of the
creatures.
Q. What succeeded the loss of the favor and image of God?
(Gen. 3:14)
A. The wrath of God and original sin.
Q. What is original sin? (Rom. 5:19; 7)
A. The corruption of our whole nature
...
Q. In what did their salvation stand?
A. In the remission of their sin and repairing of God's
image.
Q. What followed upon the repairing of God's image? (Rom.
7:5)
A. A continual battle both within and without.
Q. From whence does this battle proceed?
A. From the two contrary images in mankind.
Q. What are these images?
A. The image of God and the image of the serpent (
Reformed
Confessions of the 16th and 17th Centuries in English
Translation [Grand Rapids, MI: Reformation Heritage
Books, 2012], vol. 3, pp. 545, 546, 549).
Canons of Dordt
(1618-1619):
III/IV:1. Man was originally formed after the image of God.
His understanding was adorned with a true and saving
knowledge of his Creator, and of spiritual things; his heart
and will were upright; all his affections pure; and the
whole man was holy; but revolting from God by the
instigation of the devil, and abusing the freedom of his own
will, he forfeited these excellent gifts; and on the
contrary entailed on himself blindness of mind, horrible
darkness, vanity and perverseness of judgment, became
wicked, rebellious, and obdurate in heart and will, and
impure in his affections.