If you want to know how to delete all photos from iPhone, then read the post here.

1. Visit the App Store and install CleanMy®Phone on your iPhone. 
2. Open the app and allow authorization by clicking on the Scan button.
3. Scroll down and click on the Declutter button to start scanning.
4. Select the categories: Blur, Duplicates, and Screenshots. 
5. Now, tap on the Clean button and choose Delete from the pop-up.