For people's image being used without their permission: strengthen U.S. right of publicity laws with private right of action, enabling people to sue for unauthorized use of their voice or likeness.
Digital signatures as part of audio/video that can't be easily modified or faked which can trace the origin of a piece of media. Some camera manufacturers are already working on it.
How do you propose to keep watermark-free models out of the hands of evildoers? I can't build my own digital camera or laser printer, but I can certainly write software.
I don't have a good solution, but maybe legislation helps. There may not be a foolproof solution but I think the more that such devices are widely used, the less likelihood there may be of e.g. a court case hinged on bad evidence.