The two left hand boxes of the email text editor show the default base copy for the email for reference (colored in orange in the screenshot below).
The right hand side - colored green below - show you the version for your community which you can edit.
There's two versions of each email for every email template:
Text only email (a.k.a. "Plain Text Email"): This is the basic 100% variant which is shown as the fallback if the receiving email client cannot or does not want to show HTML formatted emails
HTML email: This is the HTML markup enhanced version which can include formatting, images and clickable links
So in order to add your header image right at the beginning inside of the <body> tag you need to edit the HTML in the bottom right field.
NB: You need to edit every email template you want this header image to show up in. To do so select each template from the " Select Email Template" dropdown and click "View".
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