$bim['bim_tthumb_type'] = new \IPS\Helpers\Form\Select( 'bim_tthumb_type', $item->upload_thumbnail ? 'upload' : 'fromURL', FALSE, array( 'options' => $options, 'toggles' => $toggles ) );
Change to
$bim['bim_tthumb_type'] = new \IPS\Helpers\Form\Select( 'bim_tthumb_type', $item->upload_thumbnail ? 'upload' : 'tthumbnail', FALSE, array( 'options' => $options, 'toggles' => $toggles ) );