

If you get the document from another source, like a customer or a supplier, the chances are that your document already has differing headers or footers for the pages in a section or that the document already has multiple sections and so on.

If you want the letterhead on all but the first page you can only use the second option. You can put it into the body text of the first page or you can change the section format to have different headers and footers on the first and subsequent pages. The other subtlety is more intrusive: let's assume that you want the letterhead only on the first page. For one, the graphics object should be behind the text, such that it doesn't get in the way of other header or page content.

This is exactly what Word does automatically, when the user clicks the Format | Background menu item. The first thought that came to my mind was putting a graphics object (the letterhead) into the header or footer of the document. The other day I was asked by a client of mine to create a convenient macro for adding watermarks or letterheads to Word documents.
