That's a bug that's been in at least the last three versions of Word (and is still there in the Word 2013 Preview).
The workaround is this:
Remove the "page background" that you've put in.
Open the header pane.
Insert the image, set its text wrapping to Behind Text, and drag its sizing handles so it covers the whole page. You may want to use the controls on the Picture Tools tab to reduce the contrast and increase the brightness.