code: function inlinePng(rootFolder, currentFile, $) {
      var currentDirectory = path.dirname(currentFile);
  
      return editHTMLElement($, 'img', function($img) {
          var src = $img.attr('src');
          if (!LocationUtils.isDataURI(src)) {
              return;
          }
  
          // We avoid generating twice the same PNG
          var hash = crc.crc32(src).toString(16);
          var fileName = hash + '.png';
  
          // Result file path
          var filePath = path.join(rootFolder, fileName);
  
          return fs.assertFile(filePath, function() {
              return imagesUtil.convertInlinePNG(src, filePath);
          })
          .then(function() {
              // Convert filename to a relative filename
              fileName = LocationUtils.relative(currentDirectory, fileName);
  
              // Replace src
              $img.attr('src', fileName);
          });
      });
  }
docs: Convert all inline PNG images to PNG file
  
  @param {String} rootFolder
  @param {HTMLDom} $
  @return {Promise}