Owners can restore deleted photos to an iPhone by syncing the phone to a backup file via iTunes or iClouds, as of April 2015. This restores only the photos synced with the last backup, as there is no way to recover photos deleted directly off of the phone.
To restore deleted photos via iTunes, the owner needs to connect the iPhone to the computer that holds the backup file and choose the Restore From Backup option. This prompts the user to choose from the available backup files and apply one to the phone. The iCloud backup is only available on phones running iOS 7 or later, and it is accessed from the Manage Storage menu of the iCloud section in the Settings app.