Get image attachment to a post outside the loop?

try using get_attached_media function and your current $postID

$images = get_attached_media( 'image', $postID );

this will return a WP_Post object. If you want to get the url, it’s located in the guid key of the object. Use a foreach loop to retrieve them all.

foreach( $images as $image ){
    $src[] = $image->guid;
}

To use a different size image, try

foreach( $images as $image ){
  $src[] = wp_get_attachment_image_src( $image->ID, 'thumbnail' )[0];
}