J'ai regardé pour faire cela pour la prochaine version.
C'est plus simple que prévu.
Il faut remplacer le code:
if(!in_array($file_type,array('file','watermark')) && JFile::exists(  $uploadPath .'thumbnail'.DS. $old)){
						JFile::delete( $uploadPath .'thumbnail'.DS. $old );
					}par:
jimport('joomla.filesystem.folder');
					$thumbnail_folders = JFolder::folders($uploadPath);
					foreach($thumbnail_folders as $thumbnail_folder){
						if(!in_array($file_type,array('file','watermark')) && JFile::exists(  $uploadPath .$thumbnail_folder.DS. $old)){
							JFile::delete( $uploadPath .$thumbnail_folder.DS. $old );
						}
					}dans administrator/components/com_hikashop/classes/product.php