...
<pd4ml:header
watermarkUrl="watermark.gif"
watermarkOpacity="50"
watermarkBounds="10,10,100,30" />
...

...
PD4ML html = new PD4ML();
...
PD4PageMark header = new PD4PageMark();
header.setWatermark( "images/logo.gif", new Rectangle(10,10,120,25), 50 );
html.setPageHeader( header );
...