$('body').on('click', '.post_content a>img, .post_content img', function(e){}

$('body').on('click', '.post_content a>img, .post_content img', function(e){}

$('.post_content a>img, .post_content img').attr('rel', 'gallery');

<script type="text/javascript">
$(function(){
    $('.post_content a>img,.post_content img').each(function(){
        var t = $(this), p = t.parent();
        (!p || !p.is('a')) && t.wrap('<a href="'+t.attr('src')+'" />');    
    });
    $('.post_content a:has(img)').attr('rel','gallery').fancybox();
});
</script>