I have a xxx web like xvideos for example. And I want to add a class to my thumbail images because I want to indicate that the pop up add only will active when they click in these images.

So, i have tried to add with the plugin woody snippets these codes:

1:

if ( has_post_thumbnail() ) { 
  the_post_thumbnail('medium', array('class' => 'alignleft'));
} 

2:

function add_image_class($class){
    $class .= ' additional-class';
    return $class;
}
add_filter('get_image_tag_class','add_image_class');

But they dont work, and at moment I dont find how to give to all my images one CLASS. Somebody can help me