Thank you for using our products Quality Construction and we are very happy that you are using support forum for the query.
The slider images are the post image of that selected category. Check all the post of that category and the post is missing featured image or not.
Example: If you have a Post named “My First Post” and that post need the featured image. If that post is set on the Slider Category and you select that category for the slider, the slider will have the image.
For the better help, you can use the dummy data to make the site and you can learn how theme works as well. Please visit the theme documentation for how to import the dummy data. http://www.doc.canyonthemes.com/quality-construction/