filter content to rewrite image src to https

force-image-https.php

<?php
add_filter( 'the_content', 'rkv_force_https_images', 10 );

/**
 * check image URLs and force https on them
 *
 * @param [type] $content [description]
 */
function rkv_force_https_images( $content ) {
	return str_replace( 'src="http://css-tricks.com', 'src="https://css-tricks.com', $content );
}