How to add a class to the attachment images

You could do like this:

function image_tag_class($class) {
    $class .= ' my-custom-class';
    return $class;
}
add_filter('get_image_tag_class', 'image_tag_class' );

in case you wanna know more about actions and hooks.

Difference Between Filter and Action Hooks?
add_filter
add_action