Thomas Aquinas

“ In the first sense the Son is the Image of the Father; in the second sense man is called the image of God; and therefore in order to express the imperfect character of the divine image in man, man is not simply called the image, but "to the image," whereby is expressed a certain movement of tendency to perfection. But it cannot be said that the Son of God is "to the image," because He is the perfect Image of the Father. ”

“ 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. ”

“ Some explain this by the fact that the Son agrees with the Father, not in nature only, but also in the notion of principle: whereas the Holy Ghost agrees neither with the Son, nor with the Father in any notion. ”