Some people misunderstand the concept of being made in God's image to mean that God modeled our physical attributes after his own. This is a mistake as Jesus clearly taught that God is not physical but a spirit (John 4:24). As I've explained elsewhere, bearing the image of God means that humans are fundamentally different from every other animal created on the earth. Part of the imago Dei is the capability we have to reason and the ability to exercise our free will and make meaningful choices.
Recently, though, asked a question that I expect many other Christians may have about this definition. A person asked "What about those who are mentally ill, though? How can they bear God's image if they lack the ability to reason or make decisions for themselves?" This is a good question that reveals bias of our modern culture that has larger implications across a variety of moral issues.
More Than a List of SkillsToday, much of what is valued in society is based on "what can you do for me" or "what skills do you have" mentality. So, it may be natural for people to assume that the imago Dei is measured by one's ability to reason, thus the question above about the mentally impaired. But one isn't considered a person because of one's ability but by nature of being human. We are designed to reflect God's image in ourselves and the design doesn't change even if we cannot properly execute the elements implicit in that. For example, a car is a vehicle whose design and purpose is to move across land, while a boat is a vehicle whose deign and purpose is to move across the water. The can may have a broken axle or the boat a hole in its side that prevent it from executing its normal function, but no one would look at a boat with a hole and say that it changes its function. Boats cannot move across the land unaided because so doing is counter to all of its design. The vehicle may need repair but one can quickly see whether it is a land or sea vehicle.
The reason this concept of design and purpose (what's known in philosophy as the telos or end purpose of a thing) is that it is crucial to the dignity of all human persons. It is not merely the mentally-ill who cannot reason, but the embryo has not yet developed reasoning capacity either. If the imago Dei doesn't apply to the embryo, then why should Christians oppose abortion? However, if the telos of the embryo is a functioning, rational adult who can make free decisions and can have a spiritual sensitivity, then the embryo shows as much uniqueness as any other human being. It is human nature to be social, to be creative, to be relational, to be rational, to have a sense of the moral, and to be spiritual. All of these reflect God's character and all sit in distinction to other animals in creation. And every being that so reflects God's image in this way is intrinsically valuable because God values these things.