Add to your functions.php file to search your content for images without an alt attribute. If they don't have an alt attribute this function will add it in with a default of the post title.

functions.php

/*-----------------------------------------------------------------------------------*/
/* ADD ALT TAGS TO IMAGE
/* ----------------------------------------------------------------------------------*/

function rdm_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', 'rdm_add_alt_tags', 99999);