I took a photo a few weeks back while on holiday in my native Africa. The windows were closed and the room was not dusty. I used my digital camera and i saw a circular blue orb in the distance in the photo at the other end of the room.
Personally i think our guardian angel(s) can appear in photos as orbs. But i definately think they r spirit beings choosing to appear to us in such form. I could be wrong but that is what i think - and im a Chrisitan 2.