Background image/watermark positioning problem

HI,

I want a image or a word to be appear as watermark or background of all the pages but place centrally.

I tried
<fo:block  font-family="Arial"
                                  font-size="12pt" text-align="center"
                                  background-image="url(com/xmltopdf/convertor/image/confidential.gif)"
                                  background-repeat = "no-repeat"
                                  background-position ="center"
                                  >
                <xsl:apply-templates/>
       </fo:block>
where background-image is confidential.gif image but i am not able to place the image at the center of the page.

i tried background-position, background-position-horizontal, background-position-vertical

thanks
imran