Church of Santa Maria dell'Ammiraglio ("The Martorana")
The inscription merely labels this person as "Saint James," but he is probably St. James the Greater, who does tend to get more attention in the art than James the Less. In the Orthodox tradition James rarely appears in the pilgrim's gear so common in the West, but this image does give him a book, common in his western portraits, rather than the scroll that is more usual in East.
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Read more about St. James the Greater.
Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.