add written signature in preview to mail signature

Take a picture of a signature

• Choose Preview > Preferences, click Signatures, and then click Create Signature.
  If a signature already exists, you can choose the Create Signature command from the Signature pop-up menu in the Annotations toolbar.
• Follow the instructions displayed on screen.
  Make sure your signature fills the box on the left and sits on the blue baseline.
• When the signature preview looks correct, click Accept.

Note this image will make your signature larger. Some users will not appreciate the bloat especially users on the MacBook Air with the smaller drives.

In Mail > Preferences > Signatures, drag the image into the signature.

You might need to resize the image.

http://osxdaily.com/2016/07/21/create-image-email-signature-mail-mac/

You might find this helpful if you want to place your image online so it's not downloaded in every email.

http://matt.coneybeare.me/how-to-make-an-html-signature-in-apple-mail-for-el-cap itan-os-x-10-dot-11/