Possible Clues:

• Sistine Chapel feature
• Certain street art
• Painting made by male artist enthralling university student
• Leonardo's 'The Last Supper,' e.g
• Subway wall art, perhaps
• Art painted on a wall
• Work that may be paid for by funds raised at Philadelphia's Wall Ball
• Banksy's 'Girl With Balloon,' e.g
• Flower completes Monet's first artwork
• Unframed artwork