Better way to create signature for preview than holding a paper in front of the camera I have a perfectly fine scanned image of a signature.  It is unnecessarily inaccurate  to hold it in front of the camera instead of using the image directly.

How to enter the signature from the image file directly?
It can't be done directly.

Best authority I can find to back up this statement is Linc Davis, on the Apple Support Communities. - https://discussions.apple.com/thread/3667416

It is possible to create one that way in Acrobat Pro/DC, but I'm not then certain if that would then also work in Preview. It would depend if Acrobat creates the signature as a true Keychain Signature item.

It's not clear from Acrobat Help / 
Sign PDF documents but implies Adobe use their own cloud service to keep tabs on signatures rather than the Keychain.