How about delete the pictures from "Camera Roll"? Is it still the same thing?
If it is still the same thing, I suppose you may need a tool to make it. You can consider iMyFone Umate Pro which is an iPhone data eraser. It can help you to permanently delete private data from iPhone. Photos are inclusive.
I suppose you should have a try.
Not only the existing photos, but also the previously deleted photos can be found out and permanently deleted. It is 100% unrecoverable. Hope this help you.