Thank you for contacting us and I'd be happy to assist.
I've checked your website's page and it is using the classic/legacy editor view. There is no built-in image slider gallery available when using the classic/legacy views, however, since the Bootstrap 3 library is being loaded on the website, you can make use of its image carousel feature: hidden link
For a different type of image gallery slider, you'll need to include a third-party plugin and here are some articles that review a few of the popular options: hidden link hidden link
I hope this helps and please let me know if you need any further assistance around this.
Yes, an alternative to the classic/legacy views editor is the view block, which can be used, if you're using the WordPress Gutenberg (blocks-based) editor.