How to add automatic class in image for wordpress post

Question

I want to make a responsive theme with Bootstrap 3. However, I need to automatically add the CSS class .img-responsive to every post image because I need the i

Answer 1

//all classes i need in a string

$classes = 'img-responsive attachment-post-thumbnail size-post-thumbnail wp-post-image'; 

//then i use my variable

the_post_thumbnail('post_thumbnail', array( 'class' => $classes ));