Add a Fancybox automatically to post type “post” images

Here is the code i use in my site, which loads all images under an anchor tag to fancybox

add_action('wp_footer','auto_fancy_box');
function auto_fancy_box(){
    if (!is_single() && !is_page())
        return;
    ?>
    <SCRIPT TYPE="text/javascript">
    jQuery(document).ready(function(){
        jQuery(".post_content").find("a:has(img)").addClass('group');
        jQuery(".post_content").find("a:has(img)").attr('rel','group1');
        jQuery("a.group").fancybox({'transitionIn':'elastic','transitionOut':'elastic','speedIn':600,'speedOut':200,'overlayShow':false});
    });
    </SCRIPT>
<?php 
}

just change the .post_content to your post content container jQuery selector.