function lowlight_intro(id)
{
	replace_background(id, 'a' + (id+1) + '.jpg');
}

function highlight_intro(id)
{
	var ext = '.jpg';
	if (id == 8)
	{
		ext = '.png';
	}
	replace_background(id, 'b' + (id+1) + ext);
}

function replace_background(id, filename)
{
	var elem = document.getElementById('intro_' + id);
	elem.style.backgroundImage = "url('images/intro/" + filename + "')";
}