function image_is_loaded(img) {
 if (!img.complete)
  return false;
 else if (typeof img.naturalWidth!="undefined" && img.naturalWidth==0)
  return false;
 else
  return true;
}

function img_src(src) {
 return folder + 'images/header/' + src;
}

function rotate_header_images() {
 var delay = 10;
 var min_opacity = 10;
 if(image_is_loaded(img)) {
  for(var j=100;j>=min_opacity;j--) {
   setTimeout('set_transparency("header-image",'+j+')',(100-j)*delay);
  }
  setTimeout('document.getElementById("header-image").style.backgroundImage="url("+img_src(images[i])+")"',(100-j)*delay);
  for(var k=min_opacity;k<=100;k++) {
   setTimeout('set_transparency("header-image",'+k+')',(k-min_opacity)*(delay)+(100-j)*delay);
  }
 }
 i++;
 if(i>=images.length) {
  i = 0;
 }
 img.src = img_src(images[i]);
}

function set_transparency(id,opacity) {
 var style = document.getElementById(id).style;
 style.opacity = opacity/100;
 style.filter = 'alpha(opacity=' + opacity + ')';
}