Protocol relative image paths in Drupal 7

custom.module

<?php

/**
 * Implementation of hook_preprocess_image().
 *
 * Make images that use a full url be protocol relative.
 */
function custom_preprocess_image(&$variables) {

  // If the image URL starts with a protocol remove it and use a
  // relative protocol.
  $scheme = file_uri_scheme($variables['path']);
  $protocols = array('http', 'https');
  if ($scheme && in_array($scheme, $protocols)) {
    $variables['path'] = '//' . file_uri_target($variables['path']);
  }
}