Likeness to the species or figure is not sufficient for the true conception of an image; there must also be similarity of origin, that from one thing another may proceed similar to itself in species, or at least in the sign of the species; and inasmuch as whatever relates to Procession and Origin in the Divinity is Personal, the term "Image" is Personal, and proper to the Son.
The Greek Doctors attribute it also to the Holy Spirit, but the Latins to the Son only, of Whom we read "that He is the Image of the invisible God, the Firstborn of every creature" (Colossians 1:15), and the "brightness of His glory and the figure of His substance" (Hebrews 1:3).
The Holy Spirit, indeed, by His Procession, receives the Nature of the Father equally with the Son, yet He is not said to be born; and in like manner, although He receives the same Species as the Father He is not called His Image. For the Son proceeds as Word, to the conception of Whom there belongs a similitude of Species to the Principle from which He proceeds; while the conception of Love does not involve that of similitude or birth as Son, but rather a going forth by some manner of impulsion; hence Love is not said to be born; nevertheless the Holy Ghost, inasmuch as He is Divine Love, resembles the Father and the Son. The Son is called the Image of the Father because He is a perfect Image of the same Nature, while man is said to be "to the Image," which expresses some motion tending towards perfection.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni