Hornby Island Rocks - for those who have not yet seen holes turn into pebbles: place my postcard on a table; rotate the image half a turn slowly, clockwise or counter-clockwise -- you might find it helpful to shift your eyes away from the image as you move it, then focus on the image again when movement has stopped. Complete the rotation and look again.
Apparently the change from holes to pebbles has something to do with the location of the shadows. Good luck! Just a little bit of fun!