Default featured image for post

Hi

I am using WP Show Post Plugin to show my post in grid and using lite version of Generatepress theme. I want to set default featured image(fallback image) for posts which user missed to add/set featured image.

I have tried following code:

<?php if ( has_post_thumbnail() ) {
the_post_thumbnail();
} else { ?>
<img src="<?php bloginfo('template_directory'); ?>/images/my-image.jpg" alt="<?php the_title(); ?>" />
<?php } ?>

But because of the plugin’s classes/function I guess I am not able to achieve results.