How to make a full-page background picture in MS Word?

Can I print a full-page background image in MS Word 2007? I have an image of a form, and want to mail-merge responses onto the form.

So I thought I would have the form as a background, and place merge fields on the appropriate spots.

Two approaches have failed.

1)‘Insert Picture’, set ‘Text Wrapping’ – ‘Behind Text’
That looks good, until I type. The picture is not ‘Behind Text’, it’s in-line. The picture gets pushed down the page as I type.

2) Custom Watermark
The image behaves correctly, it’s immobile, but it’s gray, not black.

I want to do this in Word because I have Mac and PC users who all have Word installed.