ovrPic = new Image;
ovrPic.src = 'images/subnav_icon_book_o.gif';
outPic = new Image;
outPic.src = 'images/subnav_icon_book.gif';

function swImg(imgNme, imgState)
{
  if (document.images)
  {
    if (imgState == 1)
    {
      document[imgNme].src = ovrPic.src;
    }
    else if (imgState == 0)
    {
      document[imgNme].src = outPic.src;
    }
  }
}