Filter images sent to editor to make the URLs protocol-relative for possible SSL

pilau_protocol_relative_image_urls.php

<?php

add_filter( 'image_send_to_editor', 'pilau_protocol_relative_image_urls', 999999 );
/**
* Filter images sent to editor to make the URLs protocol-relative for possible SSL
*/
function pilau_protocol_relative_image_urls( $html ) {

	// Replace protocols with relative schema
	$html = str_replace( array( 'http://', 'https://' ), '//', $html );

	return $html;
}