function rand( min, max ) {
    // http://kevin.vanzonneveld.net
    // +   original by: Leslie Hoare
    // +   bugfixed by: Onno Marsman
    // *     example 1: rand(1, 1);
    // *     returns 1: 1
    var argc = arguments.length;
    if (argc == 0) {
        min = 0;
        max = 2147483647;
    } else if (argc == 1) {
        throw new Error('Warning: rand() expects exactly 2 parameters, 1 given');
    }
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

headerImg = "assets/lay/topHeader/"+rand(1,14)+".jpg";
document.getElementById("topHeader").src = headerImg;