Objection 1. It would seem that the image of God is to be found in irrational creatures. For Dionysius says (Div. Nom. ii): "Effects are contingent images of their causes." But God is the cause not only of rational, but also of irrational creatures. Therefore the image of God is to be found in irrational creatures.
Objection 2. Further, the more distinct a likeness is, the nearer it approaches to the nature of an image. But Dionysius says (Div. Nom. iv) that "the solar ray has a very great similitude to the Divine goodness." Therefore it is made to the image of God.
Objection 3. Further, the more perfect anything is in goodness, the more it is like God. But the whole universe is more perfect in goodness than man; for though each individual thing is good, all things together are called "very good" (Genesis 1:31). Therefore the whole universe is to the image of God, and not only man.
Objection 4. Further, Boethius (De Consol. iii) says of God: "Holding the world in His mind, and forming it into His image." Therefore the whole world is to the image of God, and not only the rational creature.
On the contrary, Augustine says (Gen. ad lit. vi, 12): "Man's excellence consists in the fact that God made him to His own image by giving him an intellectual soul, which raises him above the beasts of the field." Therefore things without intellect are not made to God's image.
I answer that, Not every likeness, not even what is copied from something else, is sufficient to make an image; for if the likeness be only generic, or existing by virtue of some common accident, this does not suffice for one thing to be the image of another. For instance, a worm, though from man it may originate, cannot be called man's image, merely because of the generic likeness. Nor, if anything is made white like something else, can we say that it is the image of that thing; for whiteness is an accident belonging to many species. But the nature of an image requires likeness in species; thus the image of the king exists in his son: or, at least, in some specific accident, and chiefly in the shape; thus, we speak of a man's image in copper. Whence Hilary says pointedly that "an image is of the same species."
Now it is manifest that specific likeness follows the ultimate difference. But some things are like to God first and most commonly because they exist; secondly, because they live; and thirdly because they know or understand; and these last, as Augustine says (QQ. 83, qu. 51) "approach so near to God in likeness, that among all creatures nothing comes nearer to Him." It is clear, therefore, that intellectual creatures alone, properly speaking, are made to God's image.
Reply to Objection 1. Everything imperfect is a participation of what is perfect. Therefore even what falls short of the nature of an image, so far as it possesses any sort of likeness to God, participates in some degree the nature of an image. So Dionysius says that effects are "contingent images of their causes"; that is, as much as they happen [contingit] to be so, but not absolutely.
Reply to Objection 2. Dionysius compares the solar ray to Divine goodness, as regards its causality; not as regards its natural dignity which is involved in the idea of an image.
Reply to Objection 3. The universe is more perfect in goodness than the intellectual creature as regards extension and diffusion; but intensively and collectively the likeness to the Divine goodness is found rather in the intellectual creature, which has a capacity for the highest good. Or else we may say that a part is not rightly divided against the whole, but only against another part. Wherefore, when we say that the intellectual nature alone is to the image of God, we do not mean that the universe in any part is not to God's image, but that the other parts are excluded.
Reply to Objection 4. Boethius here uses the word "image" to express the likeness which the product of an art bears to the artistic species in the mind of the artist. Thus every creature is an image of the exemplar type thereof in the Divine mind. We are not, however, using the word "image" in this sense; but as it implies a likeness in nature, that is, inasmuch as all things, as being, are like to the First Being; as living, like to the First Life; and as intelligent, like to the Supreme Wisdom.
Source: Summa Theologica (New Advent)