The term "image of God" occurs three times in the Bible. In Genesis 1:26-27 and 9:6, we find out that man is created in the image of God. In 2 Cor. 4:4 we see the phrase used in reference to Jesus who is the "image of God." There is no exact understanding of what the phrase means, but we can generalize. It would seem that the first two verses refer to God's character and attributes that are reflected in people. The term cannot be a reference to a physical appearance of God since Jesus says in John 4:24 that God is Spirit, and in Luke 24:39 Spirit does not have flesh and bones. Therefore, we can conclude that the image of God deals with humanity's reflection of God in such things as compassion, rationality, love, hatred, fellowship, etc. God exhibits all of these characteristics, as do people.
As a side note, in 1 Cor. 11:7 man is considered the image and glory of God and the woman is the glory of man. But, we know from Genesis 1:27 that both male and female are created in the image of God.
The third verse, 2 Cor. 4:4, is specifically in reference to Christ. It says, "in whose case the god of this world has blinded the minds of the unbelieving, that they might not see the light of the gospel of the glory of Christ, who is the image of God." We know that Jesus was in the likeness of God (Phil. 2:5-8) and is the exact presentation of the nature of the Father (Heb. 1:3). Therefore, we can understand why Paul says that Christ is the image of God. But Jesus is a perfect image of God, where we are not. Jesus never sinned (1 Peter 2:22) and always did the will of the Father (John 6:37-40).
There are different opinions among Christians as to exactly what the image of God is, and to what extent it was affected by the sin of Adam. Undoubtedly, the image of God continued after the fall (Genesis 9:6), which states that whoever sheds man's blood, by man his blood shall be shed because he was made in the image of God.
Finally, because everyone is made in the image of God, then everyone should be treated with proper respect and honor because they reflect God. Of course, we know that many people are full of evil and hatred and we must guard ourselves and others against them. Still, we are to treat others with respect.