That might not be an httrack problem.  Sometimes my browser will not download
complete images either.  This usually happens to me when I run more than one
window/tab at a time.  For example, if I try to open five pages from one site
at the same time in different windows, large images on these pages will often
download only partially.

It seems to me that this is intentionally set by the webmaster to prevent DoS
attacks.  If an IP address tries to hit too many times in a particular span of
time, that IP address is temporarily stopped from "hogging" the bandwidth.  In
your position I would try limiting the number of connections and the bandwidth
of the connections (flow control and limits, respectively) to see if the
images start to download completely.  It will take longer, but it won't put a
big strain on the server.