Just to add to the other answer. When you define a background image on a module, this variable needs to be added inline and not in the CSS file as that will not work.
for example instead of using a class. Use the variable in the module.html
your code is fine. I guess the probleme is the URL, you need the absolute URL, you can't use ".." at this point, this will just be added to the url, for example "https://XXX.com/../images/heros/hero-putple.jpg". To get to the root path you just "/".