add_alt_tags

/*
 * Add Missing Alt Tags To WordPress Images : paulund.co.uk
*/
function add_alt_tags($content){
    global $post;
    preg_match_all('/<img (.*?)\/>/', $content, $images);
    if(!is_null($images)){
        foreach($images[1] as $index => $value) {
            if(!preg_match('/alt=/', $value)) {
                $new_img = str_replace('<img', '<img alt="'.$post->post_title.'"', $images[0][$index]);
                $content = str_replace($images[0][$index], $new_img, $content);
            }
        }
    }
    return $content;
}
add_filter('the_content', 'add_alt_tags', 99999);