wp_get_attachment_image() not displaying post images even though wp_get_attachment_url() echo correct urls

When saying “not working” you should include what you expect and what you get; “not working” says very little about the problem you have. Without knowing what the exact problem you have, I guess you don’t see the image. In that case I suggest to change this:

wp_get_attachment_image($photo_id, 'thumbnail');

With:

echo wp_get_attachment_image($photo_id, 'thumbnail');

wp_get_attachment_image() returns the HTML image element, but doesn’t print it.