The authors give the url for the Merode Altarpiece that the art history students put
together. The group is private and cannot be viewed by the readers of the book.

This especially unfortunate because an example is needed to be able to visualize how
the collaborative function being described works.

Author's note: The example was public at the time of writing, but Flickr allows users
to make photos private at any point. I agree that it's unfortunate that
the example has been made private, but that's one of the hazards of
writing about a changing landscape.