function changeImg(imgNumber) {
var myImages = ["bg/food01.jpg", "bg/food02.jpg", "bg/food03.jpg"];
var imgShown = document.body.style.backgroundImage;
var newImgNumber =Math.floor(Math.random()*myImages.length);
document.body.style.backgroundImage = 'url('+myImages[newImgNumber]+')';
}
window.onload=changeImg;