Well, I was thinking that the new function definition would look something like this:
$header_img = get_the_post_thumbnail( $post_id, 'post-thumbnail' );
$header_img = explode( '" class="', $header_img);
$header_img = $header_img[0];
$header_img = explode( 'src="', $header_img);
$header_img = explode( 'title="', $header_img);
$header_img = $header_img[1]; // only the url
But I don’t know enough PHP to figure out how to redefine this in a child theme. (Josh….?)