Fwiw, you can, and as a matter of "good secretarial/word processing practice" probably should (to avoid confusing others), use the image formatting options – rather than changing the page's margins completely – to move an image independently of the body of the document's text (though obviously if no one else will ever work on the document, that's not really relevant.) When you insert an image, Word will automatically shift to "format" mode, and you'll see a plethora of formatting options that will let you do what you want (and a lot more) without actually changing "the margins". It sounds like the "align" option might be enough to do what you want (setting it to "align to page" rather than "align to margin") but if that's not fine enough control, also take a look at the the "position" options, and/or the "text wrapping" options, too.
