Ghosts are the imagined spirits of people who have died. Many people claim that they have seen ghosts. Sane people can sometimes Hallucinate. Innkeepers tourist guides and others sometimes make much of stories that a place is haunted. That attracts paying visitors to the site. In one case an aparent ghost recorded on Video camera turned out to be a small insect or spider. 
It’s silly but it’s not harmless because it frightens people.
The picture is of a supposed ghost photographed by Captain Hubert C. Provand, first published in Countrylife magazine, 1936. This photograph is considered the best ghost photograph ever taken although the picture could be a deliberate fake or an accidental double exposure. Is it even clearly a picture of a woman if viewers don't expect to see a woman? Do those who believe in ghosts have nothing better?