You can add a filter to the content directly on functions.php and use str_replace or perhaps some preg_replace (regex) to change the output of the content. Not tested. Try something like this:
function add_class_to_img_and_p_in_content($content) {
$pattern ="/<img(.*?)class=\"(.*?)\"(.*?)>/i";
$replacement="<img$1class="$2 newclass newclasstwo"$3>";
$content = preg_replace($pattern, $replacement, $content);
$pattern ="/<p(.*?)class=\"(.*?)\"(.*?)>/i";
$replacement="<p$1class="$2 newclass newclasstwo"$3>";
$content = preg_replace($pattern, $replacement, $content);
return $content;
}
add_filter('the_content', 'add_class_to_img_and_p_in_content',11);
If your p and img tags do not already have a class attribute you can simply do:
function new_content($content) {
$content = str_replace('<img','<img class="newImgclass"', $content);
$content = str_replace('<p>','<p class="newPclass">', $content);
return $content;
}
add_filter('the_content','new_content');