I'm pretty sure I know where this rumor comes from.  But before
I explain it, I'd like to emphasize that the pictures are NOT OF 
KATE BUSH!

The British edition of Penthouse once had a pictorial of a someone
they called Kate Simmons (I think).  She looked superficially a little
like Kate Bush -- perhaps that was why they used her.  Later, pictures
from that pictorial turned up on Kate Bush's bootleg album "Dreamtime".
Many people, therefore, think that Kate Bush posed nude at one time.
SHE DID NOT!