The English word "image" in the Greek is like our word icon. Now you know from your computer that an icon is a little picture that represents a larger program. In the same way, Jesus is a man who represents the fullness of God. He not only represents God, He is God.
Another way the word was used back then was as a written detailed description of someone's appearance. Today we do this by putting pictures on things like driver’s licenses, passports, student IDs, whatever. But back then, a legal document might include a section where a person was described in terms of height, size, hair and eye color, and other distinguishing marks. Paul is saying that everything we see in Jesus is what and who God is.