However, looking at various people's web galleries I see a lot of personal pictures, including kids. And I'm wondering whether my caution is justified, or may merely be mild paranoia.
Definitely paranoia, unless you post your real-world address, phone number, etc to go along with them. Posting a photo of your child is fine. But I wouldn't post a map of your house showing where his bedroom is along with GPS coordinates. Posting a photo by itself doesn't really help a would-be molester or kidnapper, unless it is accompanied by additional information that would enable them to locate your child. THAT is what you should be paranoid about--posting vacation or travel plans in advance, your residential address, information about your household security precautions, daily schedule, routine travel routes.
As an educational exercise, try playing the role of someone who wishes to harm you, and research yourself online to see what information would be helpful if you wanted to break into your house and kidnap or kill a family member. Odds are that family photos would be one of the less useful tools for an attacker.