Much like news articles, in order to use galleries and/or slideshows on your site, you will need to first create them in the Admin Area of the CMS (see the Galleries page for more information). To do this, you will need to upload images to the file manager, then create a gallery using these images.
To add a gallery to a particular webpage, you will need to edit the page, then under the controls option in the Blue Bar, choose galleries then show gallery. You will then be presented with multiple options about what formatting you would like the gallery to display in, including box grid view, slideshow view etc. If you instead choose galleries then show galleries, you will have the option to display a number of galleries as thumbnails. This is useful if you need to display multiple galleries in one place.