#tblCertData thead tr th span { 
  padding-right: 25px;
  background-repeat: no-repeat;
  background-position: right center;
}

#tblCertData thead tr th.headerSortUp span{
  background-image: url('up-arrow.png');
  background-repeat:no-repeat;
}
#tblCertData thead tr th.headerSortDown span{
  background-image: url('down-arrow.png');
  background-repeat:no-repeat;
}