Can you help with post thumbnails in Wordpress?

Question

WordPress

Hello,
Can you please tell me how to display post thumbnails?
Now it looks not very good for me, to put it mildly, here is a screenshot

And here is the code:

<div class="news news_slider">
<?php 
$arg_cat = array(
'orderby'      => 'name',
'order'        => 'ASC',
'hide_empty'   => 1,
'exclude'      => '',
'include'      => '',
'taxonomy'     => 'category',
);
$categories = get_categories( $arg_cat );
?>

<?php if ( have_posts() ) while ( have_posts() ) : the_post(); ?>
  <div id="post-<?php the_ID(); ?>" <?php post_class('g post'); ?>>
      
  </div>
<?php endwhile; ?>
<?php 
if( $categories ){
foreach( $categories as $cat ){
$arg_posts =  array(
'orderby'      => 'name',
'order'        => 'ASC',
'posts_per_page' => 9,
'post_type' => 'post',
'post_status' => 'publish',
'cat' => $cat->cat_ID,
);
$query = new WP_Query($arg_posts);
?>
<?php if ($query->have_posts() ) ?>
<h2><?php echo $cat->name; ?> </h2>
<?php while ( $query->have_posts() ) : $query->the_post(); ?>
<div class="main-page-post">
  <a href="<?php the_permalink(); ?>">
  <?php the_post_thumbnail( 'widgetfull' );?>
  </a>
  <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
</div>
<?php endwhile; wp_reset_postdata()?>
<?php		
  }
  }
?>
</div>

.
Ideally, everything should look like this:

How to make everything as ideal as possible?