Insert the image into the Header (or Footer) of the document. So long as you
don't have any Unlinked sections, this will appear in all pages of the
document automatically. If you do have unlinkes sectiosn you can insert it
again for that specific section.
See
http://sbarnhill.mvps.org/WordFAQs/AnchorToHeader.htm for further details.
Remember, an image in or anchored to the header are always dimmed when
working in the document body and vice versa.
To have a document uneditable by users, you will have to look into using the
Form or Protection functions of Word that will leave certain areas open to
edit and others not. Usually these features have the Header/Footers areas
protected (against edit) but please check this.
Hope this helps
DeanH