The slider has the ability to show the post’s featured image as its background image. In the Graphene Options > Slider Options > Slider display style > Background image and excerpt.
You can then create posts under a specific category, say “Slider Images”, and assign the images you want to show in the slider as the Featured Image of those posts. Then make sure you assign that category as the category to be shown in the slider in the theme’s options page.
This way the slider will show the featured images as its background image, along with the title of the post, its excerpt, and a read more button. If you just want it to show those images, you can use CSS display: none;
on all those other elements.