Because of Node Gallery's flexible architecture, there are several possibilities how to include slideshows for your image galleries. The easiest will probably be to install the dedicated Node Gallery Slideshow module, which will automatically add a slideshow similar to the one shown in the screenshot below to your gallery pages.

The alternative solution is to use one of the views based slideshow modules available, such as Views Galleriffic. You can find a comparison of similar modules on this drupal.org handbook page.