You just need to couple the keys to something tangible. But the truth is that every system can by bypassed if you spend enough effort. The only question is when it becomes enough hassle so that it doesn't disrupt ordinary people's lives. Not even facebook cared about enforcing https until someone made a browser plugin that let everyone steal cookies.

What? No. The signature needs to be added by the image sensor as it gathers data.

It shouldn't be easy to extract it and that's it. It's even less difficult than keeping the DVD/Blueray keys secure because each device has a separate key, so if a line of devices gets compromised easily it's easy to spot.

Then you put a legal framework around what can be presented by media, the requirement for signature collection and so. And one of problems with photo/video authenticity is essentially solved.