Objection 1. It would seem that "likeness" is not properly distinguished from "image." For "genus" is not properly distinguished from "species." Now, "likeness" is to "image" as genus to species: because, "where there is image, forthwith there is likeness, but not conversely" as Augustine says (QQ. 83, qu. 74). Therefore "likeness" is not properly to be distinguished from "image."
Objection 2. Further, the nature of the image consists not only in the representation of the Divine Persons, but also in the representation of the Divine Essence, to which representation belong immortality and indivisibility. So it is not true to say that the "likeness is in the essence because it is immortal and indivisible; whereas the image is in other things" (Sent. ii, D, xvi).
Objection 3. Further, the image of God in man is threefold--the image of nature, of grace, and of glory, as above explained (4). But innocence and righteousness belong to grace. Therefore it is incorrectly said (Sent. ii, D, xvi) "that the image is taken from the memory, the understanding and the will, while the likeness is from innocence and righteousness."
Objection 4. Further, knowledge of truth belongs to the intellect, and love of virtue to the will; which two things are parts of the image. Therefore it is incorrect to say (Sent. ii, D, xvi) that "the image consists in the knowledge of truth, and the likeness in the love of virtue."
On the contrary, Augustine says (QQ. 83, qu. 51): "Some consider that these two were mentioned not without reason, namely "image" and "likeness," since, if they meant the same, one would have sufficed."
I answer that, Likeness is a kind of unity, for oneness in quality causes likeness, as the Philosopher says (Metaph. v, Did. iv, 15). Now, since "one" is a transcendental, it is both common to all, and adapted to each single thing, just as the good and the true. Wherefore, as the good can be compared to each individual thing both as its preamble, and as subsequent to it, as signifying some perfection in it, so also in the same way there exists a kind of comparison between "likeness" and "image." For the good is a preamble to man, inasmuch as man is an individual good; and, again, the good is subsequent to man, inasmuch as we may say of a certain man that he is good, by reason of his perfect virtue. In like manner, likeness may be considered in the light of a preamble to image, inasmuch as it is something more general than image, as we have said above (Article 1): and, again, it may be considered as subsequent to image, inasmuch as it signifies a certain perfection of image. For we say that an image is like or unlike what it represents, according as the representation is perfect or imperfect. Thus likeness may be distinguished from image in two ways: first as its preamble and existing in more things, and in this sense likeness regards things which are more common than the intellectual properties, wherein the image is properly to be seen. In this sense it is stated (QQ. 83, qu. 51) that "the spirit" (namely, the mind) without doubt was made to the image of God. "But the other parts of man," belonging to the soul's inferior faculties, or even to the body, "are in the opinion of some made to God's likeness." In this sense he says (De Quant. Animae ii) that the likeness of God is found in the soul's incorruptibility; for corruptible and incorruptible are differences of universal beings. But likeness may be considered in another way, as signifying the expression and perfection of the image. In this sense Damascene says (De Fide Orth. ii, 12) that the image implies "an intelligent being, endowed with free-will and self-movement, whereas likeness implies a likeness of power, as far as this may be possible in man." In the same sense "likeness" is said to belong to "the love of virtue": for there is no virtue without love of virtue.
Reply to Objection 1. "Likeness" is not distinct from "image" in the general notion of "likeness" (for thus it is included in "image"); but so far as any "likeness" falls short of "image," or again, as it perfects the idea of "image."
Reply to Objection 2. The soul's essence belongs to the "image," as representing the Divine Essence in those things which belong to the intellectual nature; but not in those conditions subsequent to general notions of being, such as simplicity and indissolubility.
Reply to Objection 3. Even certain virtues are natural to the soul, at least, in their seeds, by reason of which we may say that a natural "likeness" exists in the soul. Nor it is unfitting to us the term "image" from one point of view and from another the term "likeness."
Reply to Objection 4. Love of the word, which is knowledge loved, belongs to the nature of "image"; but love of virtue belongs to "likeness," as virtue itself belongs to likeness.
Source: Summa Theologica (New Advent)