The Independent reports that Princess Diana's brother, Earl Spencer, has purchased a drawing by artist Wolfe Lenkiewicz that shows a horse's skeleton with the head of Queen Elizabeth I "stamping on top of the crown of Queen Elizabeth II."
(2012 update: Sorry, the Independent link apparently no longer works, but I'm leaving the link in the post in case the article comes back online.)
I'm guessing that this is the image in question. Not that it necessarily matters, but the horse has wings and appears to be standing on the crown, not stamping. The picture is called "The Fairy Queen." Maybe it symbolizes, um… I don't know. You tell me.
You'll find the other interesting, creepy images by the artist here. (2012 UPDATE: That's not the link I originally posted, which has expired.)
Thank you to Dawn for sending this link!