$(document).ready(function() {
    var myarray = new Array(8);
    myarray[0] = "images/01.jpg";
    myarray[1] = "images/02.jpg";
    myarray[2] = "images/03.jpg";
    myarray[3] = "images/04.jpg";
    myarray[4] = "images/05.jpg";
    myarray[5] = "images/06.jpg";
    myarray[6] = "images/07.jpg";
    myarray[7] = "images/08.jpg";

    var number = Math.ceil(8 * Math.random()) - 1;

    $("#flash").css("background-image", "url(" + myarray[number] + ")");
});