Complex category display with thumbnail

Greetings, I have been working on a site that requires a very complex category display, it’s a product catalog and it is organized in categories and sub-categories.

The category pages have to display all posts organized by sub-category, something like:

Main Category
Sub-Cat1
Post1
Post2
Sub-Cat2
Post3
Post4

So far, thanks to help from guys at this forum, we have managed to make that work by using the following code:

<?php
//get terms (e.g. categories or post tags), then display all posts in each retrieved term
$taxonomy = 'category';//  e.g. post_tag, category
$current_cat = get_query_var('cat');
$param_type = 'category__in'; //  e.g. tag__in, category__in
$term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'child_of' => get_query_var('cat')
);
$terms = get_terms($taxonomy,$term_args);
if ($terms) {
  foreach( $terms as $term ) {
    $args=array(
      "$param_type" => array($term->term_id),
      'post_type' => 'post',
      'post_status' => 'publish',
      'showposts' => -1,
      'caller_get_posts'=> 1,
      'orderby'=> 'title',
      'order'=> 'ASC',
      );
    $my_query = null;
    $my_query = new WP_Query($args);
    if( $my_query->have_posts() ) {

      echo '<div class="gs12col" id="cat_title" >
<h4>' . $term->name. '</h4>
</div> '; 

      while ($my_query->have_posts()) : $my_query->the_post(); ?>
	<div class="gs480" id="postclass">
	<div class="gs2col" id="thumbnail">
	<a href="<?php the_permalink() ?>"><?php the_post_thumbnail( array(140,140) ); ?></a>
	</div>
	<div class="gs4col" id="postcontent">
	<a href="<?php the_permalink() ?>"><h5><?php the_title(); ?></h5></a>
	<?php the_excerpt(); ?>
	</div>
	</div>

       <?php
             endwhile;
    }
  }
}
wp_reset_query();  // Restore global post data stomped by the_post().
?>

Now, I need to display an image thumbnail and I want to use this code:

<?php
foreach((get_the_category()) as $category) {
    echo '<img src="https://example.com/images/' . $category->cat_ID . '.jpg" alt="' . $category->cat_name . '" />';
}
?>

Found here at the CODEX

However, this does not work properly, since it does not identify the correct cat_ID, it uses one of the sub-category’s ID instead of the main category.

I think the solution is to combine both codes so that as it identifies the main category to get all sub-cats, it can also define the image’s cat_ID as the main category. I don’t know how to do this, I’ve been trying but no luck.

Any ideas?

Thanks in advance! I know it was a long post, thanks for taking the time to help!