One way to do this is to create a fixed horizontal width to the display
area. I design for an 800 pixel width and present this in the horizontal
middle of the screen. So, no matter what size the visitor has his screen
resolution set to, it will be in the cventer of his screen in horizontal
placement. It will always be in the center of the screen at 800x600 and on
any resoltion that is wider.
Then I make my background image 800 pixels wide, if I dont want it to
"tile". Or, I use CSS to tile it only horizontally or only vertically.
The challenge for me was to get CSS to presnet that layer in the horizontal
middle of all screen on all browsers. But, I got it done.