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 from This iPhone
If using iCloud Photos, go to Settings
Tap your name
Tap iCloud
Tap Photos
Turn off iCloud Photos if you do not want deleted photos to sync across devices
Repeat the delete steps on any other synced Apple devices if needed