If you're willing to only support newer browsers, you can experiment with the background-size attribute:

Code:
HTML:
<div class="background"></div>
CSS:
HTML, BODY {margin: 0; padding: 0; width: 100%; height: 100%}
.background {height: 100%; width: 100%; background: url(myimage.jpg) no-repeat; background-size: 100% 100%}
That'll stretch the image, you can also try setting background-size to "cover" or "contain".

Or, if you want to support older browsers, you can stretch an image tag:

Code:
HTML:
<div class="background">
<img src="myimage.jpg"/>
</div>

CSS:
HTML, BODY {margin: 0; padding: 0; width: 100%; height: 100%}
.background {height: 100%; width: 100%;}
.background IMG {height: 100%; width: 100%; display: block}