わかる方向けコピペセット
function.phpに下記を記述
add_filter('post_thumbnail_html', 'remove_media_attribute', 10);
add_filter('image_send_to_editor', 'remove_media_attribute', 10);
function remove_media_attribute($html)
{
$html = preg_replace('/(width|height)="\d*"\s/', "", $html);
$html = preg_replace('/class=[\'"]([^\'"]+)[\'"]/i', '', $html);
return $html;
}