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$('a img').hover(function() {
  $(this).attr('src', $(this).attr('src').replace('_off', '_on'));
}, function() {
  if ( !$(this).hasClass('current') ) {
    $(this).attr('src', $(this).attr('src').replace('_on', '_off'));
  }
});
//<a href="#"><img src="btn_01_off.png" alt="ボタン" /></a>