Perhaps optional size arguments in the IMG tags? Like: [img width=320px height=240px]
I admit, I don't really know how these things work, but that's how I'd do it.
That would change the display size of the image, but not the actual file size. If the original image was, say, 3200 by 2400 pixels, you would still have to download all that data before the browser could resize it to 320 by 240. So it would eat up bandwidth and download time just the same, even if it made the display more bearable.
I think the better solution, though, would be to automatically spoiler images or convert them to links when they are quoted. But that would take quite a bit of work, I think.
That said, if you, personally, are okay with the download a large image, but display it smaller tactic, you could look into setting up a user stylesheet for your browser. “blockquote.bbc_standard_quote img {max-width: 320px; max-height: 240px; }” should automatically constrict quoted images for you.