Objection 1. It would seem that image is not said personally of God. For Augustine (Fulgentius, De Fide ad Petrum i) says, "The Godhead of the Holy Trinity and the Image whereunto man is made are one." Therefore Image is said of God essentially, and not personally.
Objection 2. Further, Hilary says (De Synod.): "An image is a like species of that which it represents." But species or form is said of God essentially. Therefore so also is Image.
Objection 3. Further, Image is derived from imitation, which implies "before" and "after." But in the divine persons there is no "before" and "after." Therefore Image cannot be a personal name in God.
On the contrary, Augustine says (De Trin. vii, 1): "What is more absurd than to say that an image is referred to itself?" Therefore the Image in God is a relation, and is thus a personal name.
I answer that, Image includes the idea of similitude. Still, not any kind of similitude suffices for the notion of image, but only similitude of species, or at least of some specific sign. In corporeal things the specific sign consists chiefly in the figure. For we see that the species of different animals are of different figures; but not of different colors. Hence if the color of anything is depicted on a wall, this is not called an image unless the figure is likewise depicted. Further, neither the similitude of species or of figure is enough for an image, which requires also the idea of origin; because, as Augustine says (QQ. lxxxiii, qu. 74): "One egg is not the image of another, because it is not derived from it." Therefore for a true image it is required that one proceeds from another like to it in species, or at least in specific sign. Now whatever imports procession or origin in God, belongs to the persons. Hence the name "Image" is a personal name.
Reply to Objection 1. Image, properly speaking, means whatever proceeds forth in likeness to another. That to the likeness of which anything proceeds, is properly speaking called the exemplar, and is improperly called the image. Nevertheless Augustine (Fulgentius) uses the name of Image in this sense when he says that the divine nature of the Holy Trinity is the Image to whom man was made.
Reply to Objection 2. "Species," as mentioned by Hilary in the definition of image, means the form derived from one thing to another. In this sense image is said to be the species of anything, as that which is assimilated to anything is called its form, inasmuch as it has a like form.
Reply to Objection 3. Imitation in God does not signify posteriority, but only assimilation.
Source: Summa Theologica (New Advent)