Email Users defines two filters which would let you do what you want. As you are seeing, by default Email Users uses the settings for the web site to define a default email template.
- mailusers_html_wrapper: This filter lets the site define the HTML used to wrap the email content. Instead of using the default settings to construct a textual header, you could define a filter that includes your header artwork.
- mailusers_manipulate_headers: This filter lets you modify the content of the email headers themselves. In theory, you can change the text of any of the headers to meet your specific needs.
Of these two filters, you should be able to do what you want with the mailusers_html_wrapper filter as it modifies the HTML. There is a post on my site that should get your started creating a filter with your specific content in it.
