function get_random_number ( max )
{
   var rand_no = Math.random();

   rand_no = rand_no * max;
   rand_no = Math.ceil(rand_no);

   return rand_no;
}

function set_main_img ()
{
   var imgs_array = new Array();

   imgs_array[0] = "img_main.jpg";
   imgs_array[1] = "img_main2.jpg";

   var img_num = get_random_number ( imgs_array.length );

   //alert(imgs_array[img_num-1]);

   document.getElementById('space').style.background = "url(/media/images/"+ imgs_array[img_num-1] +") no-repeat";
}