I'm not too experienced with httrack and only used it twice so far but I do
have experience with web development so I'll just throw out there my thoughts
about this.

Even though websites have improved from loading a whole page every time you
click a link or press a button to being able to loading things on the same
page like image galleries or showing more results on the same page etc.
everything is still the same at the core as it used to be.

And by this I mean when you click that "show more" button, even though the url
bar hasn't changed, it actually does fetch the content from somewhere.. So
depending where, often it is the same old "Prev 1 2 3 4 5  Next" system where
those links at the bottom to browse pages is replaced with a single button
that increments/loads the next page "magically" without the browser going
anywhere different.

So try looking at the page source and see if you can find a url like
<http://www.example.com/view?page=123> if you find that then you can fetch the
images directly with it and you can get the next image manually by
incrementing the image/page number.

Hope it helps..