When creating a buttons image slide, you need to add the images.
In the parent slide topic, include a file object for each button in consecutive paragraphs, and then apply the ClickableImage style to all of those paragraphs.
For the slide to function properly, you must have the same number of images as element topics. During publishing, Author-it matches the images to the element topics in the same order: the first image is matched to the first child element topic, and so on.
You saw an example of button images earlier when you learned about course slides.