Every e-mail template has a "Header" and a "Footer" section. The "Header" is the section that contains your logo at the top of the e-mail. The "Footer" is the section that contains your e-mail signature, including your headshot, signature.
In order to ensure that the Header and Footer are consistent across your templates, you can't edit change them in the individual message templates.
Instead, you need to edit the Master Header/Footer templates. When you edit these special templates, that change will be reflected across all of your e-mail templates. That way you only need to make the change once, to update.
To access the master Header/Footer templates, go to the e-mail campaigns tab and scroll down to the bottom of the page, and click on the link.
Or you can click these direct links below.