Could it be that HTTrack isn't using a "Referring URL" when it tries to grab
those images ?
I don't see anything in the HTTrack manual or FAQ that mentions "referer" or
"referring URL".

For instance I've had other sites that I try to get data from that won't send
the page unless I set a referer (spelled funny on purpose) in the headers.
This is not using HTTrack though.

If I look at "Page Info" using Firefox on this site's page, I see which page
is used as the referer.

You might be able to test this theory with wGET and it's options...

`--referer=url'
    Include `Referer: url' header in HTTP request. Useful for retrieving
documents with server-side processing that assume they are always being
retrieved by interactive web browsers and only come out properly when Referer
is set to one of the pages that point to them.