Show Featured Image on Posts

Hi
I have a featured image for my posts but it is not showing. I have tried adding the following code to the code snippets plugin
<?php
if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.
the_post_thumbnail();
}
?>
but it hasnt helped.
I dont want to have to modify the theme directly. Is there a way to show the featured image on all posts using css or snippets?
Thanks in advance for yur help!