.CHM help compiled using RHCL differs from same help compiled using RoboHelp GUI

I am using RoboHelp HTML 11.0.3.268.

My project uses graphics for list bullets in headings, set in the .css, as in the following example:

     h2 {

          list-style-image: url("img/black.png");

     }

When I view the help in the following scenarios, the graphics are displayed except for when the help has been built using RHCL:

• Previewing the topics in RH displays the graphics.
• Opening the topics in a browser (Firefox, Chrome, or Internet Explorer) displays the graphics
  
• Compiling the .chm file in RH using SSLs > Generate displays the graphics.
• Compiling the .chm using RHCL uses ordinary round bullets instead of the graphics.

Any ideas, please, how I can create the .chm in an overnight script with the graphics for bullets instead of the round ones?

Andy