if thumbnail design

I’m trying to ad a link with a class to my thumbnails but I don’t want the link and css to show if there’s is no thumbnail available, not it still shows the “frame” for the picture, since it’s a permalink. (see the posts without a thumbnail: http://www.josefinfundin.com/ )

this is my code
<a href="<?php the_permalink() ?>" class="thumb-preview"><?php the_post_thumbnail( 'thumbnail' ); ?> </a>

I’ve tried to build an if function where the <a code is called if the_post_thumbnail exist.. but unsuccessfully

help please 🙂