Here are some images which allow us a better look at the throne, which is also called the Tronetto, since it used to be set up permanently in the Sala del Tronetto, the room before the Pope's Library in the Apostolic Palace where he receives Heads of State.
As I mentioned, this throne, on which the handing over of the keys of the Kingdom of Heaven to St. Peter is depicted, was used by the popes until John Paul I for formal portraits.
Pius XI:
Pius XII:
Bl. John XXIII:
Paul VI:
John Paul I:
And lastly, this is how the throne looked until recently, when it was in the Lateran museum, until Pope Benedict (even before the advent of Guido Marini) ordered it to be restored:
