I decided to delete all of my iPhone's photos so I copied everything to iPhoto and deleted everything on my phone. But I still worry if somebody could recover my deleted photos from my iPhone? Does this method completely erase my photos on iPhone?
Open Albums view in Photos by tapping the icon in the lower-right corner. Now find the album called Recently Deleted (it will have an icon with a circled trash can).
Open it, tap Select in the upper-right corner, then Delete All in the lower-left corner. Your photos will be permanently deleted from your iPhone.