How do I attach new images to a node?
You can try to merge the existing field items array and the new one, but I would use a loop to add one image at a time:

$node = Node::load($node_id);
foreach ($new_images as $new_image) {
  $node-&gt;field_car_picture[] = $new_image;
}
$node-&gt;save();


If you want to merge the arrays get the existing one

$existing_images = $node-&gt;field_car_picture-&gt;getValue();


combine it with the new array and replace the fields content with it.