The pope is most often depicted in art wearing his crown, the Tiara, called the triple crown. It is no longer used by the Popes.
Another symbol that can be used is a key. Here Jesus gives the keys to the Kingdom to Peter. The Popes who succeed Peter also share in the Power of the Keys.
This painting is found on the wall of the Sistine Chapel. Click on the image or here to see the whole painting by Perugino. Michelangelo also uses a key to indicate Peter in the Last Judgment. For a good detail look at our page Saints at the Last Judgment.
G.G.Sill reports that there is a Papal Cross which is a Latin Cross with three unequal cross pieces. Sill, Symbolism in Christian Art, p. 31. (The three coss-pieces may represent the three areas of responsibility which all the ordain share with Christ, leader of worship, teacher, community leader. The Pope has the greatest responsibility for these missions in Christ.)