I wondered the same thing Moth.

Grasping at straws I came to the conclusion that it's a slightly unusual interpretation of the category 'Artistic/interpretive'. My best guess was that the photos are of stones look like something else (in other words, they have been interpreted as something else).

This photo seems to be similar:

http://www.themodernantiquarian.com/post/38929

Sorry if this is entirely incorrect Jo-anne.

K x