This should do it:
add_action(\'admin_notices\', \'https_the_content\');
function https_the_content() {
global $post;
if(!$post->post_content)
return;
//change src to use the current url scheme
$post->post_content = str_replace(array("src=\\"http://", "src=\'http://"), array("src=\\"//", "src=\'//"), $post->post_content);
return $post->post_content;
}