Do you know who can see the photos you upload to Facebook? Scott Hanselman has a great post about the misconceptions his wife (and many more people I would guess) had about who can see her photos when she marks them with Friends and tags a couple of people.
Who can see the photo of me and my 3 friends? Who can see the photo of my wife and her friend when the photo is marked Friends?
Answer: The union of all the friends of everyone tagged in the photo. If someone else sees the photo and tags some more people, the circle of visibility for that photo or post expands.
Scott has some great tips on other privacy settings that you should check out and make sure everyone in your family knows about.