I've been in doctor's appointments where the nurse is quizzing me on my family medical history and I've had to stop them with, "We're not biologically related and I don't know the biological history". No need for them to know the whole story.
I feel privacy for kids is really important, for a number of different reasons. (preventing kids from being exploited for likes by foster parents, preservation and ownership for them to tell their story in their way when they are ready for it, privacy, sometimes protection from relatives that don't have their best interests at heart, and probably a dozen other reasons I have never thought of.) The best default is really to keep everything private in my opinion, but obviously not every agency or foster parent will agree to all the same specifics as me.
But why is it a problem if someone sees a family photo and notices that one kid looks different? He could just be adopted, or a friend or something. Is it something like witness protection, the children might have abusive parents that shouldn't be able to find them?
If it's a matter of not standing out, they will stand out just as much when you meet them anyway, right?
Foster kids might have abusive parents as you surmised. They might have been removed from an unsafe home, or have relatives that were denied custody and might act on a photo.
> so that people don't know you're a foster kid unless you say so.
If you post a photo of your family and none of the children have emoji faces, why would anyone conclude that the foster child is a foster child? Because he looks different? Seems to me that emoji:ing out one child would do the opposite, draw attention to him.
EDIT: It seems we are talking about temporary arrangements, then things become clearer. Still very weird photos.