Friends and family know that if I take a picture, it may end up on Facebook or Instagram. Those who object know that they can ask me not to post a photo and I'll happily oblige, or they're welcome to step out of the photo if others want them posted. I've never had problems in this capacity.
My father-in-law does love posting very unflattering photos of others online (think mid-bite into messy food!) and I quietly untag myself from those; he's commented on my untagging before, at which point I gently pointed out that it's not a flattering photo. He hasn't stopped posting them, but he has stopped asking why I untag myself.
Given the ubiquity of digital cameras and social media, and people's varying views on both, the easiest thing to do is for someone who is opposed to having their photo taken or displayed online is to step out of the photo, or to tell the photographer friend that they don't want it posted online, at the time the photo is taken. I think it's unrealistic to expect that everyone will ask your permission before posting a photo online if you have said nothing at the time of the photo.