I don't think it's a fake picture - but I'm not convinced that it shows what people think it shows. Dust in the air, a lucky combination of shadows... could be anything.
On the whole, I tend to think that the vast majority of ghost sightings are nonsense, just like the vast majority of alien sightings. And most of the rest, I suspect, are simple misidentifications.
That said, science is based on fallibility; no knowledge is ever absolute, and what one generation "knows" to be true is often soundly refuted by the next. What if "there's no such thing as ghosts" is the phlogiston of the 20th century?
Personally, I don't think we can be certain that ghosts don't exist. I'd like to think they don't, because I kinda hope death is final and conclusive, but of course there's always the possibility that ghosts exist and simply aren't the spirits of dead people or whatnot.
(Don't get me started on aliens. It strikes me as supremely arrogant to believe we're the only intelligent life in the universe - but whether or not Earth's ever been visited is a totally different question.)