I just learned something new about Outlook 2007 -- new to me at least. I think the short answer is that you can't do it anymore.
Take a look at section "4 Newest release" and section "4.1 Html Rendering" in the following article -- "Microsoft Outlook":http://en.wikipedia.org/wiki/Microsoft_Outlook
There are also many other references to this change, among them:
"Why did Outlook 2007 remove the display of animated graphics...":http://groups.google.com/group/microsoft.p...821cf6154ab2842
Does this mean that senders of "fancy" emails now have the problem that some of their recipients (i.e., those recipients using Outlook 2007) will never
see the emails in their intended form, even if
those recipients choose to view a particular email in HTML?
Also, see this:http://www.clickz.com/showPage.html?page=3625034