How to get the post id of the attachment post created by Post Image field?

I have a form which uses a Post Image field to upload an image, which I need to parent to another existing post.

When doing this, there are 2 posts created, one named Untitled-# which is set as draft and set as the parent of the second one, of attachment type.

To get the ID of the parent post you can use the $entry['post_id'] but how would you get the ID of the attachment post?

Thank you,
Anton