Open the Photos app
Tap the picture or select multiple pictures
Tap the trash icon
Confirm Delete Photo
Go to Albums
Open Recently Deleted
Tap Select
Tap Delete All or choose specific photos and tap Delete
Confirm Delete Photo
Turn off iCloud Photos if you do not want deleted photos to sync across devices
Check other synced devices and delete the photos there if needed
Empty the Recently Deleted album on all devices using the same Apple ID