function random_image() 
{ 
image_array = new Array(); 
image_array[0] = '../imgs/nav/nav01.jpg'; 
image_array[1] = '../imgs/nav/nav02.jpg'; 
image_array[2] = '../imgs/nav/nav03.jpg'; 
image_array[3] = '../imgs/nav/nav04.jpg';
image_array[4] = '../imgs/nav/nav05.jpg';
image_array[5] = '../imgs/nav/nav06.jpg';
image_array[6] = '../imgs/nav/nav07.jpg';
image_array[7] = '../imgs/nav/nav08.jpg';
image_array[8] = '../imgs/nav/nav09.jpg';
image_array[9] = '../imgs/nav/nav10.jpg';
image_array[10] = '../imgs/nav/nav11.jpg';
image_array[11] = '../imgs/nav/nav12.jpg';
image_array[12] = '../imgs/nav/nav13.jpg';
image_array[13] = '../imgs/nav/nav14.jpg';
image_array[14] = '../imgs/nav/nav15.jpg';

count = image_array.length; 
random = Math.floor(Math.random() * count); 

document.getElementById('bgChange').style.backgroundImage = "url(" + image_array[random] + ")"; 
}