In the contexts where you need to prove an image is a real unaltered photograph (court cases, news media, science, etc) privacy is usually not a concern.
Nobody cares if your social media photos are edited, they probably are, it's fine.
I'm pretty sure if signing images becomes the norm, because cameras will do it automatically then social media sites etc will start to care - or at least be thankful for the additional metadata.
I'm pretty sure if signing images becomes the norm, because cameras will do it automatically then social media sites etc will start to care - or at least be thankful for the additional metadata.