add_action('acf/save_post', 'move_image_to_post_thumbnail');
function move_image_to_post_thumbnail($post_id) {
if (get_field('your-image-field-name', $post_id, false)) {
// get unformatted value
// we did it above too, no point formatting the value for this
$image_id = get_field('your-image-field-name', $post_id, false);
update_post_meta($post_id, '_thumbnail_id', $image_id);
}
}