code: function convertImages(images, options) {
      if (!options.convertImages) return Q();
  
      var downloaded = [];
      options.book.log.debug.ln('convert ', images.length, 'images to png');
  
      return batch.execEach(images, {
          max: 100,
          fn: function(image) {
              var imgin = path.resolve(options.book.options.output, image.source);
  
              return Q()
  
              // Write image if need to be download
              .then(function() {
                  if (!image.origin && !_.contains(downloaded, image.origin)) return;
                  options.book.log.debug('download image', image.origin, '...');
                  downloaded.push(image.origin);
                  return options.book.log.debug.promise(fs.writeStream(imgin, request(image.origin)))
                  .fail(function(err) {
                      if (!_.isError(err)) err = new Error(err);
  
                      err.message = 'Fail downloading '+image.origin+': '+err.message;
                      throw err;
                  });
              })
  
              // Write svg if content
              .then(function() {
                  if (!image.content) return;
                  return fs.writeFile(imgin, image.content);
              })
  
              // Convert
              .then(function() {
                  if (!image.dest) return;
                  var imgout = path.resolve(options.book.options.output, image.dest);
                  options.book.log.debug('convert image', image.source, 'to', image.dest, '...');
                  return options.book.log.debug.promise(imgUtils.convertSVG(imgin, imgout));
              });
          }
      })
      .then(function() {
          options.book.log.debug.ok(images.length+' images converted with success');
      });
  }
docs: Convert svg images to png