Have you tried position:fixed?
#content-background
{
display: block;
position: fixed;
width: 100%;
height: 100%;
top: 0;
left: 0;
}
possibly need to add another container
<div id="content-background">
<div id="relative-container">
<div id="img1"></div>
<div id="img2"></div>
<div id="img3"></div>
<div id="img4"></div>
</div>
</div>
#relative-container