Oh god, Facebook pictures? Don't give it a second thought. Friend should have spoken to you directly. And it's perfectly fine in many people's views.
"If I did that with my older child..." etc - bollocks. Absolute bollocks that they could know that. SOME children might be bothered by it, but most definitely not all. DH and I put together a photo album of us as children from our parents' photo stashes to pass around at our wedding and I chose some bath photos. It's just not a big deal. Some people think it is, that's their hang up. You can argue facebook is different, but it's really no different to passing them around at a family party, which you may well do.
Christ, please don't think you've "been going massively wrong for years". I have several NT friends who post nude pictures. I don't, it's a line I don't cross, but I don't think there's anything wrong with it as such. Some people seem to think that photos posted "on social media" can magically go viral and end up all over the internet. They don't. If you posted it friends only then only your friends can see it.
Often it happens that a thread starts which could go one way or another, and the first couple of posts really set the tone for the thread. If you started an identical thread at another time, you might get some quite strong responses in the other direction. MN is nowhere near as balanced and sensible as it used to be - it has been especially hysterical this week. (Half term??)