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<script>
  $(document).ready(function(){
//WHAT NUMBER TO START COUNTING FROM
var count = 0;
//LOOP THROUGH EVERY ROW
$(".contentImgRow").each(function(){
    //SET THE REL ATTR TO GALLERY PLUS COUNT FOR EACH CHILD
    $(this).find(".overlayImg").attr("rel", "gallery" + count++);
});
    });
</script>