“Image” here is not a visible form, but it refers to the inner reality of God, something of His Essence.
According to St. John’s first letter, God is love. His essence is love, a self-giving, unconditional, sacrificial love, based on a purely free act of self-emptying and receiving.
It is this “image” of true freedom and love that He poured out upon us that shaped us into His children. It is for this reason that our identity is found only as beloved children of God, and our heart can be fulfilled only in responding to and in imitating this love.