how to add thumbnails to wordpress blog posts code example

Example 1: show featured image in post wordpress

<?php if(has_post_thumbnail()){
  the_post_thumnbail('cstm_name', array( 'class' => 'class1 class2' ))
} else { ?>
  <img class="card-img-top" src="http://placehold.it/750x300" alt="Card image cap" class="img-responsive">
<?php } ?>

Example 2: add featured image

function fn_add_featured(){
   add_theme_support( 'post-thumbnails' );
   add_image_size( 'post-imge', 730, 380 );
}

add_action('after_setup_theme','fn_add_featured');