I know this might be unpopular, but have you considered whether it's just a defect in the lense? Worth taking a photo in a different room, and comparing
In terms of finding out, if you are in the UK your parish church register, and/or local library is a good place to start. your local library will have past copies of old newspapers. Asking neighbours if they know anything about a ghost, looking up your house deeds (which will have a list of the owners) and cross referencing with the list of births and deaths.
if the house is older than 50 years, there is bound to be someone who died there.