Hello,
Please note, the issue still persists with the feature addon override that comes with the Rhino template.
Since I could not wait for a solution, I removed the override and extracted the icons I needed from the rhino.svg file and used the non-overriden feature addon that comes with SP Page Builder 3.7.9 and the feature box images now appear.
This confirms my suspicion that there seems to be an issue with the site.php override that comes with the Rhino template. Rhino doesn't seem to work well with SP Page Builder 3.7.9
Hopefully this gets fixed in the next update of Rhino.
Kind regards,
John