code: function fetchRemoteImages(rootFolder, currentFile, $) {
      var currentDirectory = path.dirname(currentFile);
  
      return editHTMLElement($, 'img', function($img) {
          var src = $img.attr('src');
          var extension = path.extname(src);
  
          if (!LocationUtils.isExternal(src)) {
              return;
          }
  
          // We avoid generating twice the same PNG
          var hash = crc.crc32(src).toString(16);
          var fileName = hash + extension;
          var filePath = path.join(rootFolder, fileName);
  
          return fs.assertFile(filePath, function() {
              return fs.download(src, filePath);
          })
          .then(function() {
              // Convert to relative
              src = LocationUtils.relative(currentDirectory, fileName);
  
              $img.replaceWith('<img src="' + src + '" />');
          });
      });
  }
docs: Fetch all remote images
  
  @param {String} rootFolder
  @param {String} currentFile
  @param {HTMLDom} $
  @return {Promise}