Word's Watermark feature only applies a watermark on a per page basis.
Business Card templates are nothing more than a table (columns & rows) which
allow for multiple copies of the same content per page. If you want a
watermark on each card it has to be a part of the per card (cell) content
just like anything else on each card.
Format the graphic to the level of brightness/contrast & size needed, set
its Text Wrapping to Behind Text, then copy it & paste into each "card" on
the label document. Positioning probably will be a bit of a challenge
Quite honestly, *customized* business cards - as with most other special
layout requirements - are not best handled by a word processing program.
There are a number of page layout programs available which aren't
necessarily expensive but are designed for such purposes. One major
advantage is that you have to lay out just the one copy of the card & the
software automatically repeats it based on the number of cards per page. You
may want to do a little Googling before you proceed
HTH |:>)
Bob Jones
[MVP] Office:Mac