Note that modifications to the stock package are not supported by UBBCentral.
You'd be looking at email_text.tpl and email_html.tpl, which cover HTML and text emails. Be sure to use a full URL to any images that you include in any of the templates (as a relative URL doesn't include your domain name).
While I was talking with Isaac, he had mentioned that you could try editing one of the language strings to display an image, if that worked with the layout you were planning.
