No mannequins or costumed guides are employed at the castle, and three photographic experts have confirmed that no digital trickery was used on the photo.
Confirmed ghost sceptic Professor Richard Wiseman, who led the study, said: "It is certainly very curious. We ran it by three photographic experts and they said it hadn't been Photoshopped at all.
"The figure appears to be in period costume, but we know 100% that Tantallon Castle is not the sort of place that has dummies or costumed guides - they just don't go in for that sort of thing."
Tantallon Castle, a ruined fortress dating back to the 14th Century, stands on a remote rocky headland near North Berwick on the Scottish east coast.
Christopher Aitchison, who took the photo, said: "I was not aware of anyone, or anything, being present in my picture, only noticing the anomaly when I got home."
Psychologist Prof Wiseman, from the University of Hertfordshire, who has made many studies of the supernatural, launched the investigation a month ago.
Members of the public were asked to submit ghostly images for experts to analyse, the best of which were posted on the website www.scienceofhauntings.com.