Open the Photos app
Tap Library
Tap Select
Tap each photo you want to delete, or drag to select multiple photos
Tap the Trash icon
Tap Delete Photos
Open Albums
Scroll down and tap Recently Deleted
Tap Select
Tap Delete All
Tap Delete Photos to confirm
If using iCloud Photos, repeat on all synced devices if needed
If you want to remove photos from the iPhone without deleting them from iCloud, turn off iCloud Photos first in Settings > Photos
If you want to erase everything on the iPhone, go to Settings > General > Transfer or Reset iPhone > Erase All Content and Settings