We behold the face of the earth furnished with terrestrial creatures, and man, created after Your image and likeness, in that very image and likeness of You (that is, the power of reason and understanding) on account of which he was set over all irrational creatures. [Confessions XIII.32]
Not every likeness, not even what is copied from something else, is sufficient to make an image; for if the likeness be only generic, or existing by virtue of some common accident, this does not suffice for one thing to be the image of another. For instance, a worm, though from man it may originate, cannot be called man's image, merely because of the generic likeness. Nor, if anything is made white like something else, can we say that it is the image of that thing; for whiteness is an accident belonging to many species. But the nature of an image requires likeness in species; thus the image of the king exists in his son: or, at least, in some specific accident, and chiefly in the shape; thus, we speak of a man's image in copper. Whence Hilary says pointedly that "an image is of the same species."
Now it is manifest that specific likeness follows the ultimate difference. But some things are like to God first and most commonly because they exist; secondly, because they live; and thirdly because they know or understand; and these last, as Augustine says (QQ. 83, qu. 51) "approach so near to God in likeness, that among all creatures nothing comes nearer to Him." It is clear, therefore, that intellectual creatures alone, properly speaking, are made to God's image. [ST I, Q93, A2]