//引数に与えたURLをページ全体表示背景にする
function setFullBackgroundImage(url)
{
	if(document.body == null || document.body.style == null)
		return;
	var img = new Image();
	img.src = url;
	img.style.width = "100%";
	img.style.height = "100%";
	img.style.position = "fixed";
	img.style.borderStyle = "none";
	img.style.zIndex = 0;
	img.style.left = "0px";
	img.style.top = "0px";
	var layer = document.createElement("div");
	layer.style.width = "100%";
	layer.style.height = "100%";
	layer.style.position = "relative";
	layer.style.zIndex = 1;
	layer.style.left = "0px";
	layer.style.top = "0px";
	while(document.body.hasChildNodes())
		layer.appendChild(document.body.firstChild);
	document.body.appendChild(layer);
	document.body.appendChild(img);
}

//test.jpgを設定するテスト
window.onload = function(){setFullBackgroundImage("test.jpg");};

<img src="images/back.gif" width="100%" height="100%"
     style="position:absolute; top:0px; left:0px; z-index:1;">
<div style="position:absolute; top:10px; left:10px; z-index:2">
</div>