How to add automatic class in image for wordpress post

<?php
//----------------------------------------------------------/
//  responsive images [ 1) add img-responsive class 2) remove dimensions ]
//----------------------------------------------------------/

function bootstrap_responsive_images( $html ){
  $classes = 'img-responsive'; // separated by spaces, e.g. 'img image-link'

  // check if there are already classes assigned to the anchor
  if ( preg_match('/<img.*? class="/', $html) ) {
    $html = preg_replace('/(<img.*? class=".*?)(".*?\/>)/', '$1 ' . $classes . ' $2', $html);
  } else {
    $html = preg_replace('/(<img.*?)(\/>)/', '$1 class="' . $classes . '" $2', $html);
  }
  // remove dimensions from images,, does not need it!
  $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
  return $html;
}
add_filter( 'the_content','bootstrap_responsive_images',10 );
add_filter( 'post_thumbnail_html', 'bootstrap_responsive_images', 10 );



Wordpress Bootstrap 3 responsive images

since you need to have it for all of your post images, then you need to add a hook for the content and add

function add_responsive_class($content){

        $content = mb_convert_encoding($content, 'HTML-ENTITIES', "UTF-8");
        $document = new DOMDocument();
        libxml_use_internal_errors(true);
        $document->loadHTML(utf8_decode($content));

        $imgs = $document->getElementsByTagName('img');
        foreach ($imgs as $img) {
           $img->setAttribute('class','img-responsive');
        }

        $html = $document->saveHTML();
        return $html;
}
now add the hook to the content

add_filter        ('the_content', 'add_responsive_class');