You cannot scale a background if you defined it as part of your CSS without using some JS. In the example below, the black part of the background would scale/resize with the browser window but the image would not; the img would just happily sit @ top:0, left:0, render 1:1 and laugh at you.
/* Black will scale, images does not */
#someDiv {
display:block;
width:100%;
height:100%;
background:#000 url(someImage.jpg) 0 0 no-repeat;
}
If you need to have your background image in your CSS for whatever reason, then read this http://css-tricks.com/perfect-full-page-background-image/ for how to manage scaling CSS backgrounds w/ JS
or
If you defined your background as an img then you have a better chance and you don't even need to use JS (although you probably should if you want to maintain ratio/scale).