HTML Webpage - sections - convert data-bg-img to background image - html

Question

I bought a webpage ( files ) and on some website hosting providers it works, on others it doesn't.
And was wondering if any of you can point me in the right direction on how to change the index.html content to display the correct background image.
The webpage is static, divided into sections, having the below structure for the sidebar which hides/shows different section, with different background image.
<div class="inner">
<nav id="tmMainNav" class="tm-main-nav">
<ul>
<li>
<a href="#intro" id="tmNavLink1" class="scrolly active" data-bg-img="images/Homepage.png" data-page="#tm-section-1">
<i class="fas fa-home tm-nav-fa-icon"></i>
<span>Introduction</span>
</a>
</li>
<li>
<a href="#products" id="tmNavLink2" class="scrolly" data-bg-img="images/Portofolio.jpg" data-page="#tm-section-2" data-page-type="carousel">
<i class="fas fa-map tm-nav-fa-icon"></i>
<span>Projects</span>
</a>
</li>
<li>
<a href="#company" id="tmNavLink3" class="scrolly" data-bg-img="images/Projects.png" data-page="#tm-section-3">
<i class="fas fa-users tm-nav-fa-icon"></i>
<span>Qualifications</span>
</a>
</li>
<li>
<a href="#contact" id="tmNavLink4" class="scrolly" data-bg-img="images/Contact.jpg" data-page="#tm-section-4">
<i class="fas fa-comments tm-nav-fa-icon"></i>
<span>Contact</span>
</a>
</li>
</ul>
</nav>
</div>
How should I change "data-bg-img="images/Homepage.png" " to actually render background-image ?
I think if it was possible, would have been awesome, to put into the CSS at the top of the page, in the style, all the background images, and somehow to trigger the one for which the section is currently viewed.
The sections are called from this:
<!-- section 1 -->
<section id="tm-section-1" class="tm-section">
<div class="ml-auto">
<div class="myDiv">
<header class="mb-4"><h1 class="tm-text-shadow"><strong> greetings </strong></h1></header>
Continue
<!-- data-nav-link holds the ID of nav item, which means this link should behave the same as that nav item -->
</div>
</div>
</section>