Help Required: Assigning a class to a PHP string

Hi Guys,

I'm sure this is fairly simple for a more experienced PHP developer, basically I want to assign a class of 'alignright' to the image generated by the code below.

<?php echo get_the_post_thumbnail( get_the_ID(), array( 300, 300 ) ); ?>

Code snippet is from the WordPress Classifieds plug-in.

Any help would be hugely appreciated!