Objection 1. It would seem that the image of God is not in man. For it is written (Isaiah 40:18): "To whom have you likened God? or what image will you make for Him?"
Objection 2. Further, to be the image of God is the property of the First-Begotten, of Whom the Apostle says (Colossians 1:15): "Who is the image of the invisible God, the First-Born of every creature." Therefore the image of God is not to be found in man.
Objection 3. Further, Hilary says (De Synod [Super i can. Synod. Ancyr.) that "an image is of the same species as that which it represents"; and he also says that "an image is the undivided and united likeness of one thing adequately representing another." But there is no species common to both God and man; nor can there be a comparison of equality between God and man. Therefore there can be no image of God in man.
On the contrary, It is written (Genesis 1:26): "Let Us make man to Our own image and likeness."
I answer that, As Augustine says (QQ. 83, qu. 74): "Where an image exists, there forthwith is likeness; but where there is likeness, there is not necessarily an image." Hence it is clear that likeness is essential to an image; and that an image adds something to likeness--namely, that it is copied from something else. For an "image" is so called because it is produced as an imitation of something else; wherefore, for instance, an egg, however much like and equal to another egg, is not called an image of the other egg, because it is not copied from it.
But equality does not belong to the essence of an image; for as Augustine says (QQ. 83, qu. 74): "Where there is an image there is not necessarily equality," as we see in a person's image reflected in a glass. Yet this is of the essence of a perfect image; for in a perfect image nothing is wanting that is to be found in that of which it is a copy. Now it is manifest that in man there is some likeness to God, copied from God as from an exemplar; yet this likeness is not one of equality, for such an exemplar infinitely excels its copy. Therefore there is in man a likeness to God; not, indeed, a perfect likeness, but imperfect. And Scripture implies the same when it says that man was made "to" God's likeness; for the preposition "to" signifies a certain approach, as of something at a distance.
Reply to Objection 1. The Prophet speaks of bodily images made by man. Therefore he says pointedly: "What image will you make for Him?" But God made a spiritual image to Himself in man.
Reply to Objection 2. The First-Born of creatures is the perfect Image of God, reflecting perfectly that of which He is the Image, and so He is said to be the "Image," and never "to the image." But man is said to be both "image" by reason of the likeness; and "to the image" by reason of the imperfect likeness. And since the perfect likeness to God cannot be except in an identical nature, the Image of God exists in His first-born Son; as the image of the king is in his son, who is of the same nature as himself: whereas it exists in man as in an alien nature, as the image of the king is in a silver coin, as Augustine says explains in De decem Chordis (Serm. ix, al, xcvi, De Tempore).
Reply to Objection 3. As unity means absence of division, a species is said to be the same as far as it is one. Now a thing is said to be one not only numerically, specifically, or generically, but also according to a certain analogy or proportion. In this sense a creature is one with God, or like to Him; but when Hilary says "of a thing which adequately represents another," this is to be understood of a perfect image.
Source: Summa Theologica (New Advent)