I use the Uncode theme which allows me to place my featured image in the posts, however it only shows me those that have a large image (approximately over 1500 wide) and the small images do not show them.

In the single.php file I found this fragment:

$media = get_post_meta($post->ID, '_uncode_featured_media', 1);
$media_display = get_post_meta($post->ID, '_uncode_featured_media_display', 1);
$featured_image = get_post_thumbnail_id($post->ID);
if ($featured_image === '') {
    $featured_image = $media;
}