Forum Centering the doc_background image
 

I have this doc_background image you can see at http://www.farstrider.co/forums/forum.php .

Notice that the doc_background image is a bit to the left, how do i center it? The gap to the right and below is not visible because i also filled out background colour #000000.

On second thoughts, I would like to know how to center the image in header background as well? (not the logo or vBulletin logo, just the background image)