Ian McDowell perpetrated this one on me via a Dueling Modems post, in which he writes:

Apologies to anyone who's already gotten this lots of times from well-meaning but annoyingly-forward-happy friends. This photo purports to depict an actual ghost, that of a widow who died in this house while waiting for her husband to come back from the Civil War.

As with most such optional illusions, you have to stare at it awhile. Don't focus on any one point, but you can concentrate on the general area around the chair.