Better way to create signature for preview than holding a paper in front of the camera

It appears that at present, that isn't an option. When I created a signature file using the FaceTime camera, I see that first of all, there is no unencrypted graphic file created by Preview. Preview did a new key in Keychain titled "Preview signature privacy", suggesting that the signature files go directly from Preview's camera interface to an encrypted file. There isn't a supported way to add a graphic as a signature, and the encryption makes it difficult to bypass the approved new signature process.

The Mac users need to make sure that their outgoing email format is HTML or Rich Text even if the incoming format was plain text or use a plain text signature.

I'd prefer the latter: I use plain text email whenever possible as it's more-widely compatible and usually is easier to read. This, by necessity, would entail using plain text signatures.