How can I attach an image from an existing image field to another multi-value image field?
The first line doesn't get you an image, it's a field item list:
$items = $node-&gt;get('field_old_image_field');

Then you can get the first item
$item = $items[0];

and append the item properties to the new field:
$node-&gt;get('field_new_image_field')-&gt;appendItem([
  'target_id' =&gt; $item-&gt;target_id,
  'alt' =&gt; $item-&gt;alt,
  'title' =&gt; $item-&gt;title,
]);