var isIMG = document.images;
var Arrowon;
var Arrowoff;

if (isIMG) {
   Arrowon = new Image(7,7);
   Arrowon.src = "http://www.your-domain.com/images/arrow_over.gif";
   Arrowoff = new Image(7,7);
   Arrowoff.src = "http://www.your-domain.com/images/arrow_down.gif";
}
function Arrow(Arrowname) {
   if (isIMG) {
     document[Arrowname].src = (document[Arrowname].src.indexOf('arrow_over.gif') != -1) ? 'http://www.your-domain.com/images/arrow_down.gif' : 'http://www.your-domain.com/images/arrow_over.gif';

   }
}