code: function(callback) {
        if (imagesPaths.length >= numberOfColumns * numberOfRows) return callback();
  
        var transparentImagePath = path.join(temporaryDirectoryPath, 'transparent.png');
        gm(width, height, '#00000000').write(transparentImagePath, function(error) {
  
          // Add as many as needed transparent images to the list of images
          var totalMissingImages = numberOfColumns * numberOfRows - imagesPaths.length;
  
          for (var i = 0; i < totalMissingImages; i++)
            imagesPaths.push(transparentImagePath);
  
          callback(error);
        });
      }
docs: Complete the grid defined by numberOfColumns and numberOfRows using transparent images if needed