Something like this will work for you. You will want to expand on this function to make sure the $imagearray and several other security checks.
function my_acf_save_post( $post_id ) {
$user_id = get_current_user_id();
$imagearray = get_field('user_image','user_' . $user_id);
$imagepath = $imagearray['url'];
update_field('image_url', $imagepath);
}
add_action('acf/save_post', 'my_acf_save_post', 20);
