Add class to every image inside a <p> on the_content

img_p_class_content_filter.php

<?php
add_filter( 'the_content', 'img_p_class_content_filter', 20);

function img_p_class_content_filter($content) {
	$content = preg_replace("/(<p[^>]*)(\>.*)(\<img.*)(<\/p>)/im", "\$1 class='content-img-wrap'\$2\$3\$4", $content);
	return $content;
}