The Epistle of Barnabas is a letter written circa 70 – 130 A.D probably by an Alexandrian Jew in the period of Trajan and Hadrian. In this letter we find an early church understanding of Genesis 1:26.
For it is written concerning Him, partly with reference to Israel, and partly to us; and [the Scripture] saith thus: “He was wounded for our transgressions, and braised for our iniquities: with His stripes we are healed. He was brought as a sheep to the slaughter, and as a lamb which is dumb before its shearer.” Therefore we ought to be deeply grateful to the Lord, because He has both made known to us things that are past, and hath given us wisdom concerning things present, and hath not left us without understanding in regard to things which are to come. Now, the Scripture saith, “Not unjustly are nets spread out for birds.” This means that the man perishes justly, who, having a knowledge of the way of righteousness, rushes off into the way of darkness. And further, my brethren: if the Lord endured to suffer for our soul, He being Lord of all the world, to whom God said at the foundation of the world, “Let us make man after our image, and after our likeness,” understand how it was that He endured to suffer at the hand of men.(Epistle of Barnabas, 5:1b-10, Ante-Nicene Father Vol.I)
“For it is concerning us that the scripture says that he says to the Son, “Let us make man after our image and likeness, and let them rule the beasts of the earth, and the birds of heaven, and the fishes of the sea.” And the Lord said, when he saw our fair creation, “Increase and multiply and fill the earth”; these things were spoken to the Son. “(Epistle of Barnabas, 6:12, The Apostolic fathers)
The “us” in Genesis 1:26, as believed by early Christians, is the Father speaking to the Son.
The Ante-Nicene Fathers, Volume I: The Apostolic Fathers with Justin Martyr and Irenaeus. 1885 (A. Roberts, J. Donaldson & A. C. Coxe, Ed.) (139). Buffalo, NY: Christian Literature Company.
Clement I, P., Clement I, P., Ignatius, S., Bishop of Antioch, Polycarp, S., Bishop of Smyrna, & Lake, K. (1912-13). The Apostolic fathers (P. Clement I, S. Ignatius, Bishop of Antioch, S. Polycarp, Bishop of Smyrna & K. Lake, Ed.). The Loeb classical library. London; New York: Heinemann; Macmillan.