Do you mean that you want it to appear in every page? Or that you want a
given image to span multiple pages (ie., so that part of the image is on one
page, another part on another, etc.)?
If you mean the former, that is done by putting the watermark image into the
footer or header layer of the document. Note that something placed into the
header or footer layer of the document can actually appear anywhere on the
page -- not just at the top of bottom.
If you mean the latter, I don't know of anything in Word that will do that.