wordpress plugin preview image template code example
Example: 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 } ?>