Here is how to add an HTML email signature:
Under Mail >> Preferences
Create an empty Signature
Turn off / Close Apple Mail
In the Finder Navigation (top of screen), click on Go, while pressing "Option"; Library will appear. Clicking on the Library list item.
In the Library folder, click on Mail >>V7 (or V8 depending on your OS version) >> MailData >> Signatures >> file ending with .mailsignature
Open in Textedit
Remove unnecessary code, adding your HTML signature file between the opening and closing <body> tags.
Save + Lock the file. Quit apple mail
Re-open Apple Mail and test your signature.
These steps do not carry over if you are using icloud. You will need to redo these steps on each of your apple computers.