Header images a the top of the e-mail display properly in Outlook XP, Outlook 2000, Outlook 2003 and Outlook Express 6, but NOT Outlook 2007. What could be the problem, what should I be looking for?
This could be due to having a background image set, which isn't supported in Outlook 2007. Apart from that you'll need to provide more information if you want a better answer.
For a full guide of Email browser limitations I reccomend visiting http://www.email-standards.org/ to narrow down all possibilities.