/**
 * Add autmatically rel="prettyPhoto" attribut when you have one image in a post
 * or rel="prettyPhoto[0]" attribut when you have more than one image in a post
 */
function sumtips_image_attribute($content) {
        global $post;

        /* Search img tag or src attribut in the post content */
        $regular_expression  = '/<a(.*)href="(.*)"(.*)><img(.*)src="(.*)><\/a>/';

        /* grab all the images from the post in an array $allpics using preg_match_all */
        preg_match_all( $regular_expression, $post->post_content, $allpics );

        /* Count image in a post */
        $nb = count($allpics[0]);

        /* Replace HTML attribut */
        if ($nb == 1) {

                /* If we have only one image in a post */
                $pattern ="/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
                $replacement = '<a$1href=$2$3.$4$5 rel="prettyPhoto">';
                $content = preg_replace($pattern, $replacement, $content);
                return $content;

        } elseif ($nb > 2) {

                /* If we have more than one image in a post */
                $pattern ="/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
                $replacement = '<a$1href=$2$3.$4$5 rel="prettyPhoto[0]">';
                $content = preg_replace($pattern, $replacement, $content);
                return $content;
        }
}
add_filter('the_content', 'sumtips_image_attribute');