From The Art and Popular Culture Encyclopedia
Over the years "St. Peter" has evolved into a stock character that is now widely used in jokes, cartoons, comedies, dramas, and plays. Such caricatures almost all play upon Peter's role as the "keeper of the keys of the kingdom of heaven" in Matthew 16:19, on the basis of which he is often depicted as an elderly, bearded man who sits at the pearly gates that serve as heaven's main entrance, often portrayed as a sort of hotel-style doorman / bouncer who personally interviews prospective entrants into Heaven, often seated behind a desk, or standing at a lectern. He usually has a big book in which the names of the saved are written.
In traditional Medieval iconography, Peter is a bald man with a long beard. He usually has one or more large keys in his hand or hanging from his belt.