window.onload = function (e) {

	var content				= document.getElementById("content");
	// var links = document.getElementsByTagName("a");
	var links				= content.getElementsByTagName("a");
	var preloadableImages	= [];

	for( i=0; i<links.length; i++) {
		var l = links[i];
		
		l.onmouseover = function(e) {
			var li = e.target.parentElement;
			li.style.backgroundImage = "url(progress/" + li.id + ".png)";
		}

		l.onmouseout = function(e) {
			var li = e.target.parentElement;
			li.style.backgroundImage = "url(progress/" + li.id + "_bw.png)";
		}
		
		preloadableImages.push( "progress/" + l.parentElement.id + ".png" );
		preloadableImages.push( "progress/" + l.parentElement.id + "_bw.png" );
	}
	
	preload( preloadableImages );
}

function preload( imageUrlsToPreload ) {
	for (var i=0; i < imageUrlsToPreload.length; i++) {
		var image	= new Image();
		image.src	= imageUrlsToPreload[i];
	};
}