This is my understanding of two possible dangers of posting pictures - there might be more:
One, a picture of a very cute child or baby could be copied and used to circulate on facebook for an appeal for funds because they supposedly have cancer or some other illness or whatever. This has been known to happen.
Two, if a person can work out the name of a young person from the post, and know the town where the poster lives (this information is available on facebook if you have put it there) then a paedophile could trace the child and approach them, call them by name, claim to be a friend, knowing various things about them, and strike up a relationship.
If you have only family and close well known friends, and strong privacy settings, it is safe, but it is unbelievable how many people's facebook pages are open to the public. Even if your page is secure, one of your friends and family ( eg teenage nephews and nieces) may have unsecure pages which (I assume but am not sure about) could show your posts to them. You can choose a setting for your posts that says 'friends only' and make sure it does not say 'friends of friends'.
Also, with modern editing apps, anybody's picture can be copied and changed and made to look different. This could lead to anything. Unfortunately It is a sign of the times we live in.