Modify Featured Image markup in posts listing using wp posts pro plugin.

gistfile1.txt

/**
 * Modify featured image html.
 * @param  string $thumbnail  Default featured image markup.
 * @param  int    $post_id      Post ID.
 * @param  int    $thumbnail_id Attachment ID.
 * @return string               Modified featured image markup.
 */
function wpp_thumbnail_html( $thumbnail,$post_id, $thumbnail_id) {

	$image_attributes = wp_get_attachment_image_src( $thumbnail_id,'thumbnail' );
	if ( $image_attributes ) {
		$thumbnail = '<img src="'.$image_attributes[0].'" width="'.$image_attributes[1].'" height="'.$image_attributes[2].'" />';
	}

	return $thumbnail;
}

You can modify featured image markup using this hook. You can add your own class or modify size of the featured image.

You need to paste above code in functions.php of your activated theme.