Possible Clues:

• A retweet by past master - a crafty type who knows all about the old pictures
• Knows the origins of the great wave off Kanagawa being an expert on The Liffey Swim
• Past master could tell you things about 18 across
• Expert in the past of painters etc
• Has a view on Mona Lisa¿s smile, for starters, in 18 across with past master
• Heretic pens this prominent feature for the Courtauld's past master?