hello,
If you are using Free Version Of the Full Frame Theme. You Will have to use the Featured Page Slider, because the demo slider is just for demo purposes you cannot add or edit its images.
To use Featured Page Slider go to dashboard > Appearance > Customise. Look for the option that says “Featured Slider Options” and give it a click. Then select “Featured Slider Type” and switch it from “Demo Featured Slider” to “Featured Page Slider”. The slider will show the featured image of the page. If you haven’t posted the page’s images you will see the themes default image.
But If you want to add your own image and don’t want to use the page’s featured you will have to upgrade to Full Frame Pro
To see the awesome features of the pro version check Here
To see how to upgrade to pro version check here
Regards,
Bplv