If I was you I would just add simple styling.
Code:
body{ background-image: bgname; background-size: 100% Auto; }
The above will stretch image to 100% wide and the auto part is the height so that it maintains the aspect ratio
That way does not matter what browser, size monitor, device you are viewing the image will always take 100 % of the available space. You dont have to set it to 100% of course if you want a coloum down the center of the page you can maybe set it to 80% etc.
Centered
Code:
body {
background-image: url("http://yoursite.com/images/image_name.gif");
background-position: 50% 50%;
background-repeat: no-repeat;
}
Simplified
Code:
body {
background: url("http://yoursite.com/images/image_name.gif") 50% 50% no-repeat;
}
and so on that is the basics many free sites out there that will dive more into css styling or html body tags but as you can see all very similar