> All images of the site are missing(!) and 'external.gif' 
> is used instead. All images are simple html <img src>

> I have tried with +*.gif +*.jpg and with --assume 
> gif=image/gif but no change...

After examining the debug log file I noticed that the images have received a
"403: Forbidden" status from the server. Checking the image URL from browser
yielded the same error. However the error was gone after a while.

Apparently site's web server was suspecting something and blocking access to
httrack. So I used -u0 option (Don't check document type if unknown) and
provided document types with --assume option.

The download worked perfectly with the -u0 option! And it went faster, too, -
because the server was not spending time testing the document types...