To delete a photo permanently:
- Go to your profile and press "Photos"
- Select the album where the photo you want to delete is
- Locate the photo and press on it
- Press "Delete Photo"
Photos are permanently deleted. Once deleted, photos cannot be recovered.
Deleting a photo will make the photo disappear from all posts and messages that have the photo on them.
Keep in mind that browsers may cache images locally - see Why is my deleted photo still appearing in my post or message?