When Annie Leibovitz's picture of child star Miley Cyrus appeared in Vanity Fair her tween fans - and their parents - went ballistic. The naked back, the satin sheet, the damp hair ... how dare the innocent heroine of the hit series Hannah Montana look so provocative? Germaine Greer dissects the image itself, and we look at the people behind it: the photographer, Annie Leibovitz, and Disney, which markets the billion-dollar actor