Yes I understood perfectly Charles, hence my question about which email client you are using.
Those (clients) which can import rich text (html) templates are few and they each have their own formatting requirements.
Online (web mail) such as gmail you simply cannot use your own template.
Windows Mail uses 'Stationary' - you could try saving your page as .html and then with Windows Mail (for example) browsing Format >> Apply Stationary >> More Stationary then open the exported HTML file. Though this will only allow you to have a background of your choice in my tests.