Three Plague Saints
Church of St. Mary of Nazareth ("The Scalzi"), Venice, Italy
Sebastian and Roch are often pictured together because both were considered saints to pray to in time of plague, Roch himself having been a victim of that disease. Gonzaga is here because he too died of the plague, contracting it while tending to plague victims in a Roman hospital.
The vision to which Roch and Sebastian cast their gaze is of the heart of Jesus as it is pictured in Sacred Heart images: set in a crown of thorns, with flames and a cross at the top. Widespread devotion to the Sacred Heart began in the 17th century, so the painting must be from that century or later.
Each saint is portrayed in the canonical fashion: Sebastian tied to a tree nearly naked and pierced by arrows; Roch with a wound in his thigh, a pilgrim's staff, and a shell emblem in his breast; and Aloysius Gonzaga as a youth in cassock and surplice holding the cross.
In the plaque on the altar St. Joseph kisses the Christ Child. He is not one of the saints traditionally called on for intercession during a plague.
This image in full resolution
See the page for the Sacred Heart.
More of St. Sebastian
More of St. Roch
More of St. Aloysius Gonzaga
Photographed in Venice by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.