My girlfriend had an entire series of pictures that she took in a dorm room (not haunted by any means) with "orbs" identical to this.. on her clothing, her friends clothing, and in the air. I've edited artifacts like this out of many photos, and I've come to conclude that it IS water droplets on the lens itself. Because water is transparent AND it reflects, it causes all sorts of strange things on camera lenses; especially when it is on or near the lens itself. Sorry.
EDIT: theMarq is very correct as well.
Although, if you wanted to believe it wasn't -- there's a perfect "monster" for this artifact. The Will 'o Wisp.. created from the vapors that rise from the dense jungle swamps, these mindless forms seek out the energy contained in all living things. While not evil in nature, their feeding habits do make them a serious threat to humans and animals.
"The will o'wisp is a creature that dwells in the darkness of swamps and bogs. It can shape itself to look like a lantern or any other source of light, making it the perfect hunter - and killer."
Eep.. scary!