MBS-5447 asks for 250px thumbnails to be used to save bandwidth. If we're concerned about bandwidth, we should definitely stop automatically embedding the original images in the page when we don't yet have a thumbnail. Instead we could show a fallback image/text and if someone really wants to load the full image, they can click on it and open the lightbox.
e.g. http://musicbrainz.org/release/f1ca9290-5143-4cce-befd-517ab4492663 embeds a 3.6 MB image in the page.