What is the danger in posting pictures online?

I know some people are very vigilant on not posting any pictures of themselves online. I’ve always wondered…what is the big deal? What harm can come of it?

On a parenting site, some crazies will steal your photos, and pretend that they are their children. This bothers me only to an extent (note: this has not happened to me, but to many others) The victims usually become hypervigilant about not posting any personal information.
I would be upset, but I would not think it was a personal attack into my private life.
I personally post any picture of myself, or my son that I feel like. Is this such a bad thing?