St. Martin of Tours? St. Mark?
Church of St. Martin of Tours, Burano, Italy
The saint on the throne could be St. Mark, because of the book and the short, square beard. The painting is quite similar to Titian's
St. Mark Enthroned.
Or it could be St. Martin, the patron of the church. The figure points up with his index finger, a gesture sometimes seen in other portraits of St. Martin.
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Read more about images of St. Martin of Tours and St. Mark.
Photographed at the church by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.