Saint Peter is shown with the upside-down cross on which he was crucified. He holds a key because Jesus ordained him to become the leader of the Apostles (see Matthew 16:18-19), and thus he became the first pope in a continuous line of popes that includes the one we have today.

All Catholic priests get their ordination directly from the line of bishops that goes all the way back to Peter and Jesus (called the "Apostolic Succession"), and this is why they alone have the ordination (we call this "Holy Orders") that enables them to consecrate the bread and wine so that it becomes the actual Body and Precious Blood of Jesus himself.