function refresh_image()
{
var array_imgs = new Array("rental_trucks_1-new.jpg", "rental_trucks_2-new.jpg", "rental_trucks_3-new.jpg", "rental_trucks_4-new.jpg");
	var index = randomToN(array_imgs.length -1);
    document.getElementById("header_image").style.backgroundImage = "url(images/" + array_imgs[index] + ")";
	
}

function randomToN(maxVal,floatVal)
{   var randVal = Math.random()*maxVal;   
    return typeof floatVal=='undefined'?Math.round(randVal):randVal.toFixed(floatVal);
}