Ever get an orb (a ball of light) in a photo you’ve taken? It is possible that it is the result of dust (light hitting it), or a reflection of some kind in the lens. I would not presume that every orb is a sign of a ghost.
But it could be.
In my own experience, I’ve had orbs show up most often in places known to be haunted (e.g., San Jose’s Winchester Mystery House – I had one pic with perhaps a dozen or two dozen orbs in a place that felt haunted to me). I’ve also had a lot of orbs show up during family gatherings, particularly when there were many children present. I see that and think, “of course, they wouldn’t have missed a party when they were alive – why miss one now?”
In looking at photos of ghosts (or purported ones) on the web, I found a site that I particularly liked because of the healthy skepticism the author has, the comments on the photos and what could or likely would not be an explanation, and even the listing of the sources of information (emails from the photographers themselves). So if you’re interested in ghost pics, I would recommend that you take a look at “Ghosts and Apparitions“, a free geocities site that appears to be affiliated with The Shadowlands.