$(document).ready(function(){
  if( $.fn.pngfix != null )
    $("img[@src$=png]").pngfix();
  var tmpalt;
  $("img").hover( 
    function(){ tmpalt = $(this).attr( "alt" ); $(this).attr( "alt", "" ); },
    function(){ $(this).attr( "alt", tmpalt ); });

});