// $product


$medias = array();
foreach ($product->medias as $m) {
  $medias[] = $m->filename;
}



// in the post var, you will now have ['medias'][{filenames}]
echo $zest->image_selector('medias', $medias,TRUE,FALSE);



// then when posted you want to do:

$medias = array();
foreach ($_POST['medias'] as $m) {
  $medias[] = ORM::factory('media',$m)->id;
}

$product->medias = $medias;
$product->save();