Umm, a ghost? Are you serious? Let's consider the two possibilities as to what this could be:

1) A common lens flare, something every photographer is familiar with.
2) A disembodied representation of the memories and experiences and personality of a dead human being, which somehow transferred itself into a tiny orb floating by your head. An orb which has the strange property unknown to any other object in that it emits light visible to cameras but not to human eyes.

Seriously, how do people believe such weird things? You wouldn't believe how many images there are with lens flares on the Internet that people swear are ghosts. Let me teach you a little something about your camera: sometimes there will be smudges of light on your pictures that do not represent actual objects in front of the camera. There is no way to avoid lens flares in all your pictures; every photographer knows that lenses in certain lighting conditions will reflect light in this annoying way. There's nothing supernatural about it at all.

Plus, ghosts do not exist. Your memories and personality and all that stuff are complex arrangements of neurons firing around in your brain. Once you're dead, that's it, game over. Although I can see the appeal of pretending otherwise, there is no evidence at all that anything in nature will preserve the contents of your brain somehow after you die.

I'm actually glad that ghosts don't exist. Sometimes I like to jack off or take a shit and I don't want to think that my dead grandfather is looking down on me all the time.