How to use file_save to store pictures into a folder on server?
Try this:

$file = file_save((object)array(
    'filename' =&gt; $file_name,
    'uri' =&gt; $destination_uri,
    'status' =&gt; FILE_STATUS_PERMANENT,
    'filemime' =&gt; file_get_mimetype($destination_uri),
  ));
//at that point you can use the $file-&gt;fid
//do the magic on your custom table field_data_field_photo_de_couverture