Many attempts have been made to explain the plural forms: “Let us make man in our image, in our likeness” : e.g., (1) the plural is a reference to the Trinity; (2) the plural is a reference to God and his heavenly court of angels; (3) the plural is an attempt to avoid the idea of an immediate resemblance of humans to God; (4) the plural is an expression of deliberation on God’s part as he sets out to create the human race. The singulars in v. 27 (“in his own image” and “in the image of God”; cf. 5:1) rule out explanation 2, since in the immediate context the creation of man and woman is said to be “in his image,” with no mention of them in the image of the angels. Explanations 3 and 4 are both possible, but neither explanation is specifically supported by the context. Verse 27 states twice that “man” was created in God’s image and a third time that man was created “male and female.” The same pattern is found in Genesis 5:1-2. The singular “man” is created as a plurality, “male and female.” In a similar way the one God (“And God said”) created humankind through an expression of his plurality (“Let us make man in our image”). Following this clue the divine plurality expressed in v. 26 is seen as an anticipation of the human plurality of the man and woman, thus casting the human relationship between man and woman as a reflection of God’s own personal relationship with himself.
–NIV Bible Commentary