In the early 1930s, William Valentiner thought the Roman Baroque courtyard at the Detroit Institute of Arts needed to be spruced up. With a commission from Edsel Ford, he hired Diego Rivera to paint two large murals of "some motif suggesting the development of industry." The end result was the Detroit Industry Murals. [more inside]
In a Michelangelo Fresco, Visions of a Brain Stem. "It has been hiding in plain sight for the past 500 years, and now two Johns Hopkins professors believe they have found it: one of Michelangelo’s rare anatomical drawings in a panel high on the ceiling of the Sistine Chapel. Michelangelo was a conscientious student of human anatomy and enthusiastically dissected corpses throughout his life, but few of his anatomical drawings survive. This one, a depiction of the human brain and brain stem, appears to be drawn on the neck of God, but not all art historians can see it there."
Knossos: Fakes, Facts, and Mystery. "The masterpieces of Minoan art are not what they seem... The truth is that these famous icons are largely modern. As any sharp-eyed visitor to the Heraklion museum can spot, what survives of the original paintings amounts in most cases to no more than a few square inches. The rest is more or less imaginative reconstruction, commissioned in the first half of the twentieth century by Sir Arthur Evans, the British excavator of the palace of Knossos (and the man who coined the term 'Minoan' for this prehistoric Cretan civilization, after the mythical King Minos who is said to have held the throne there). As a general rule of thumb, the more famous the image now is, the less of it is actually ancient."
The Mills-Kronborg Collection of Danish Church Wall Paintings, courtesy of Princeton University's Index of Christian Art, includes descriptions and images of medieval and early modern church frescoes. There are more church frescoes at Painting and Sculpture in Medieval Hungary. (Another site features a fine panorama.) Anne Marshall has developed an extensive site devoted to similar paintings in England, many of which were whitewashed during the Reformation. The University of Leicester hosts a much more specialized database devoted to the Seven Deadly Sins and the Seven Corporal Works of Mercy (no images); La Mort Dans L'Art/Death in Art has some Continental examples of The Three Living and the Three Dead.