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?
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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.