I think this image behaviour has something in common to those stated in
<http://forum.httrack.com/readmsg/14307>

As suggested (and IMHO) HTTrack considers the image folder to be outside range
of depth option, that is: on 1st level pages, "image folder" stands at level 2
and thus is linked/downloaded as needed; but on level 2 pages "image folder"
is considered out of reach, in spite those images have been already
downloaded, or being in a level 2 folder (from starting page)

If wrong, I would be happy to know whats happening (I'm pretty sure it has
nothing to do with CSS/js, just check my test-website)