If you are inserting the image as a "watermark," then the colors are intentionally muted (the "washout" property is applied automatically). You can access the document header and change the color setting on your background image (which is anchored to the header). It will still be dimmed in Word when the document body is active. In addition, some wrapped images may display dimmed even when you view the header (this is a bug) but should print properly. For more on this, see
http://sbarnhill.mvps.org/WordFAQs/AnchorToHeader.htm