If the desired output of the default WordPress post thumbnail is not enough you can easily adjust the PHP to output your appropriate class.
<?php the_post_thumbnail('post-thumbnail', array( 'class' => "YOUR CLASS")); ?>
More information can be found on the codex.
Want to become a better web developer?
Join over 25,000 other developer & designers who get awesome links to the best news and articles each week delivered directly to their inbox.