I don't think HTTrack works that way. I believe what it does is scan HTML files
for links then grabs files that way.

If you've got a page that references IMGs in that folder then that's what you
need to find and start there. But I don't believe the program works like
explorer and just "gimme all files in this folder".

I've seen plenty of sites that disallow a directory listing, but will let me
download the files in that directory if I know their full name.