Output post thumbnail (add class)

Use wp_get_attachment_image() instead if you want to set a class property.

Example:

$image = wp_get_attachment_image(
    get_post_thumbnail_id(), 
    array(100, 100), 
    true, 
    array('class' => 'your-class-name')
);

//returns the HTML for the attachment
echo $image; 

The above function returns the HTML img tag:

<img src="http://example.com/path/to/image.jpg" class="your-class-name"/>

You can see the difference between functions here:

• https://developer.wordpress.org/reference/functions/wp_get_attachment_image/
• https://developer.wordpress.org/reference/functions/wp_get_attachment_image_src/