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https://developer.wordpress.org/themes/functionality/featured-images-post-thumbnails/

----------------
function.php-be:
add_theme_support( 'post-thumbnails' );

///-----------
kiiratom a kepet:

<?php if ( has_post_thumbnail() ) : ?>
  <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
  <img src="<?php the_post_thumbnail_url(); ?>"/>
  </a>
<?php endif; ?>