No one knows what most individual saints looked like. There are no photographs before the nineteenth century and usually no drawings. For this reason, artists use a symbol associated with the particular saint to identify him or her. The man in the foreground is St. Peter. We can see this because he is carrying two large keys. This is the common symbol associated with Peter. Why?
Matt. 16:18-19 "And I tell you, you are Peter, and on this rock I will build my church, and the gates of Hades will not prevail against it. I will give you the keys of the kingdom of heaven, and whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven." NRSV.
This is a very important quote. First, it predicts that the church will not be destroyed by the forces of evil and death. Evil and Hades will sometimes win individual battles, but they cannot win the war. Second, Peter's authority and power are established. The power to bind and loose in heaven is remarkable. It is the power to admit someone to heaven or exclude them; it is the power to forgive sins. This authority is shared with the other apostles and their successors. (Priests too share in the authority to forgive sins, if they act in unity with their bishop.)