Outlook 2007 will not process background images in html emails (but Outlook 2002 does).
You have to use tables for html emails. See this page item 2 for general advice.
http://www.wickham43.net/formhtmlemail.php
Html emails can be difficult because different browsers process in different ways, so it pays to keep the table code very simple.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.