Creating a default theme with a background watermark (2010)

Hello,

The company I'm working for was given the order to remake the watermark our customer had in Outlook 2003. It appeared on every new email they made.

Now, after three hours of searching, I still don't have an answer where to find the information.

So, simply said; How to make a default watermark for each new email?

Regards, MusicDemon