Add class to custom thumbnail

The third parameter you can pass to get_the_post_thumbnail is $attributes, which you can use to pass custom HTML attributes including classes.

$thumbnail = get_the_post_thumbnail(
    get_the_ID(), // Post ID.
    'thumbnail', // Image size.
    array( // Attributes
        'class' => 'my-custom class-names', // Classes go here.
    )
);

Note that this will override the default image classes that WordPress assigns (which is attachment-$size_class size-$size_class)