It is not a file it is the key field. The file is mailer.php

EMAIL_HEADER_HTML
Is the name of the field but sometimes it will not find it. I guess it depends on what browser you are using.
The language file is mailer.php.
Find the EMAIL_HEADER_HTML string and add whatever html code you want to it.
So cp>>Languages>Language editor
Choose Specific Language File: Mailer.php
Scroll down till you see EMAIL_HEADER_HTML
Edit the content.

Thank you so much Ruben, I found the line.

$ubbt_lang['EMAIL_HEADER_HTML'] = "No need to reply to this email.";

What would be the correct way to add this?

<p align="center">
<img border="0" src="http://centraltexasstangs.com/header/ctsbanner.jpg" width="800" height="100"></p>

Never mind thank you again, I got it.