if (document.images) 
{
  img_logo = new Image();
  img_download = new Image();
  img_download_focus = new Image();
  img_download_hover = new Image();

  img_logo.src = "/media/layout/masthead_logo_filled.png";
  img_download.src = "/media/buttons/download.png";
  img_download_focus.src = "/media/buttons/download_focus.png";
  img_download_hover.src = "/media/buttons/download_hover.png";
}