//List of background images
var backgroundSrcs = new Array("images/header-forest.jpg","images/header-field.jpg","images/header-beach.jpg","images/header-dog.jpg","images/header-girl.jpg")

function pickimage()
{
//Random from the list you entered above
var bgimage=backgroundSrcs[(Math.round(Math.random()*(backgroundSrcs.length-1)))]

//Applies the background image to your id
document.getElementById("image").style.background = "url('" + bgimage + "') left no-repeat #fff"; 
}