The content in the slideshow is controlled by the corresponding view named "Slideshow". This view collects all the published nodes that have the "Promoted on slideshow" field  enabled. For example the content types "Product", "Service", "Slideshow entry", "Blog entry", that come with the Business Class distributions, already implement this field .
The image uploaded for the "Teaser image" field is automatically cropped, resized and placed in the slideshow.
In order to give your content types the option to be promoted to the slideshow you should add the mentioned fields to them. You should use the "Add existing field" section to do that  in order to avoid the "... already exists" error message.