From Wikipeda:
Goatse.cx was an infamous Internet shock site that became well-known among various Internet forums because of trolls who posted links in order to upset other forum readers. Its front page featured a picture, hello.jpg, of a naked man stretching his anus open to a diameter roughly equal to the width of his hand, with the inside of his rectum clearly visible. Visible below his anus were his scrotum and dangling, flaccid, penis.