Responsive images in Drupal. The height and width attributes need removing from the <img> elements and we need to add a little CSS to allow the images to scale correctly

style.css

img {
  max-width:100%;
  height:auto;
}

template.php

<?php

// remove height and width attributes from images
// replace THEME with the name of your theme
function THEME_preprocess_image(&$variables) {
  $attributes = &$variables['attributes'];

  foreach (array('width', 'height') as $key) {
    unset($attributes[$key]);
    unset($variables[$key]);
  }
}