Thomas Aquinas

Summa Theologiae

About imago dei:

"Since human beings are said to be in the image of God in virtue of their having a nature that includes an intellect, such a nature is most in the image of God in virtue of being most able to imitate God;" [Ia q.93 a. 4]

"Only in rational creatures is there found a likeness of God which counts as an image .. As far as a likeness of the divine nature is concerned, rational creatures seem somehow to attain a representation of [that] type in virtue of imitating God not only in this, that he is and lives, but especially in this, that he understands." [Ia q. 93 a.6]