Boston Massacre Picture: Ghosts?

Old, historic and haunted, Boston is famous for ghosts and of course the Boston Massacre. Karen of Brimfield, IL sent us this nice orb picture, taken at the site of the Boston Massacre:

"I took this photo of the gravestone of the victims of the Boston Massacre (which started the Revolutionary War) in Boston, in the middle of the day during the week of June 23, 2006. It was a little overcast and drizzly. I am new to ghost sightings. Can you tell me if these are orbs? Spirits? Interestingly the images do not appear on any other photos taken that day or on that roll."

We're not sure what caused the orbs: moisture, dust or spirit.  The photo is great as it is...