Facebook claims to allow restrict sharing to friends, certain network, or even custom when I upload photos.
But it appears that this security is only in effect for the page where the photo is hosted. One can use the "Copy image URL" or the equivalent in one's browser, and you can forward this URL to anyone, who will be able to see it without even logging in to Facebook.
This looks weird, because I've seen companies without a superstar IT security team be able to restrict their images/resources with LDAP or something similar. Why can't Facebook do this?