Set class on thumbnail images in plugin list category post

I am hoping that someone can help me – How can I set the Class on the Thumbnail images?

I have looked here: In the catList.php file

 $lcp_thumbnail = '<a href="' . get_permalink($single->ID).'">' . get_the_post_thumbnail($single->ID, $this->params['thumbnail_size']) . '</a>';

The default class on the thumbnail looks like this – class="attachment-thumbnail wp-post-image"

It would be great to set the class on the thumbnail image, via the shortcode.

Thanks