Since you are storing an url for each image you can create a query to get the id of the attachment based on the guid
:
global $wpdb;
$ids = array();
foreach ($images as $image_url) {
$ids[] = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url));
}
Another way would be to simply store the ID
rather than the url in your custom field, then there would not be necessary to perform a the above query for each image.