I have struggled with mirroring a site which seems to be affected by the same
problem.

I have HTTrack (Web interface) 3.46-1 on Linux and it does not parse and
retrieve a background image in css if it is written as "background:
url(path/to/image) bla bla bla;" - their is no quotes around the path in the
parenthesis.

Hopes this makes the problem clearer.