If you have a set of images to use as cards, you can add these using the “Add Media” button at the bottom of the card list.

You can then multi-select the images in the file browser.

Each card will get a full-screen image.

You can customise slides with the popup-editor or the Card sidebar (this lets you multi-select cards to change multiple cards at once).

Alternatively: drag and drop

If you select a set of images in Finder you can drop these directly into your deck and a card will automatically be created for each.