If you ever see an illustration of a man with an X-shaped cross, it is probably St. Andrew who is traditionally believed to have been crucified on an X-shaped crucifix in Patras, Greece.
St. Bartholomew is often depicted with a knife, symbolizing his martyrdom of being skinned alive. The book represents some writings that he may have done.
St. John the Evangelist (left) is pictured holding a book, representing his writings in the Bible. Another common depiction of John is at the foot of the cross with Mary. His brother, James the Greater (right) is holding a walking staff with a scallop shell, both objects symbolizing the great pilgrimage to his place of burial - the Camino de Santiago de Compostela.
In this wall painting, St. Peter is shown with a fishnet to remind us that he was a fisherman, a key to symbolize Jesus giving him the keys to the kingdom of heaven, and the cock at his feet remind of the occasion when he denied Jesus three times. Peter is also holding a scroll perhaps to represent some Epistles possibly written by this saint (1 and 2 Peter).