This works for me:
<div style="position: fixed; top:0px; left:0px; z-index: -1; background: url(http://adserver.url/www/delivery/avw.php?zoneid=ZONEID&cb=RANDOM_NUMBER&n=NID) center center no-repeat; width: 100%; height:100%;">
<a href='http://adserver.url/www/delivery/ck.php?n=NID&cb=RANDOM_NUMBER' target='_blank' style="display: block; width: 100%; height: 100%;"></a>
</div>
You create new zone with custom size: * x *, so you can upload your background banner. Link banners to this zone. Check zone id in code up. You can have multiple banners displayed as background image.