Command Line compile misses images referenced in CSS

With Adobe RoboHelp HTML 8, building HTML Help from HTML source, I was trying to use the command-line compile feature to set up a batch script to compile a set of projects. It mostly works, with

rhcl "path to each project.xpj" -l

in the .bat file, run within the RoboHelp Command Prompt.

But I find that images referenced only in the CSS, such as the one here:

h1 { background-image: url('image/header.gif'); }

do not get compiled into the .chm file. Opening the resulting .chm files in HTML Help Studio confirms that the reason the image is now showing up is because it is not in the .chm.

These images work correctly when compiling from within the user interface. I was able to make this work by adding the files as baggage files, but then I get warnings ("this file is already in the FILES section") when I compile from within the user interface. I worked around the problem by opening and compiling each project from within an Extendscript (yay for multiple ways to do things) but this makes me question the worth of the command line compile.