Again on this one I would have to say dust. If you zoom in on the orb, and enhance the brightness and contrast to get a better picture of it you can see the coronal outline of the object. This is where the light of the flash hits the exterior of the dust particle. Also the shape of the object suggests dust or particle.
Not saying there is no paranormal activity going on there, but in my opinion the picture itself is not capturing it. Most true orbs are more radiant in nature, almost spectral.
But then again, just my opinion I would love to hear other's.