Show content based on page number

Current post page can be detected using get_query_var('page');.

note that if you do not pass any page the page 1 is shown, but get_query_var('page'); will be 0;

So in your function.php you can add:

function get_page_image() {
  global $post;
  if ( ! is_object($post) || ! isset($post->ID) ) return;
  $page = get_query_var('page');
  if ( empty($page) ) $page = 1;
  $field = 'image_' . $page;
  return get_post_meta($post->ID, $field, true);
}