Use a
<img></img>
with
position:fixed;top:0;left:0;width:100%;height:100%;
and negative z-index. There's unfortunately no way to implement this behavior in IE 8 using only CSS.
See the following article for further information:
How Do you Stretch a Background Image in a Web Page.
If you wish to use an image as a background for a given element try the following approach:
<div class="fullbackground">
<img class="fullbackground" src="yourImageSrc" />
</div>
.fullbackground{
position:relative;
}
img.fullbackground{
position:absolute;
z-index:-1;
top:0;
left:0;
width:100%; /* alternative: right:0; */
height:100%; /* alternative: bottom:0; */
}