[Plugin: Multiple Post Thumbnails] Add class to

I’m trying to add a class attribute to the <img> element, by using the function as follows in my template:
`if (class_exists(‘MultiPostThumbnails’) && MultiPostThumbnails::has_post_thumbnail(‘website’, ‘website-screenshot’)) :
MultiPostThumbnails::the_post_thumbnail(‘website’, ‘website-screenshot’, NULL, ‘medium’, array(‘class’=>’alignnone’) ); endif;`

However it is not outputting any classes, am I doing something wrong?