How to show featured image instead of post content?

Hello guys,

Maybe you can help me with a little thing...

I have a code to show a video, and when I don't have a video, the post content is shown instead. I need to replace the post content (text) with the post featured image: the_post_thumbnail('full');

This is my code:

<div id="play_video">

<?php $content=get_post_meta($videoId,"wpscript_embed",true);
if($content){                 
$width = '100%';
$content = preg_replace('/width="(.*?)"/i', 'width="' . $width .'"', $content);
$content = preg_replace('/src="http:/i', 'src="https:', $content);
echo $content; 
}else{
echo apply_filters('the_content', get_post_field('post_content', $videoId));
}
?>

<script type="text/javascript">

<?php 
$id=get_post_thumbnail_id( $videoId );
?>

var poster = '<?php echo  wp_get_attachment_url( $id ); ?>';
var x = document.getElementsByClassName("wp-video-shortcode");
if(x[0] != 'undefined' || x[0]!=''){
x[0].setAttribute('poster', poster);
}
</script> 

</div>

PHP:

I think I only need to change this line:

echo apply_filters('the_content', get_post_field('post_content', $videoId));

PHP:

If I replace the line above with this one:

the_post_thumbnail('full');

PHP:

It works... but it shows the image in both posts, with videos, and without videos.
I only want to show in posts without videos.

Can someone help me?

Thanks in advance.