Hello Community,
In the Introduction from the Interstellar Theme you can read to add Images in a Portfolio:
To create image slideshow gallery go to WP Dashboard -> Galleries -> Add New. Upload few images by clicking Add Media button. Set Featured Image. All the images you uploaded to this portfolio item will be displayed as a slideshow when you view that portfolio item page.
But in my Backend i don't find Galleries or i am blind. Maybe anyone can say me how to add Images in the Portfolio step by step.