Delete Camera Roll Images
Connect the iPhone to a Mac or PC running iTunes and do a complete Restore to factory settings. This will wipe out all media &amp; data and install the latest version of iOS for that device.

If you don't have a Mac or PC for this task, on the iPhone itself, you can go to Settings > General > Reset > Erase all content and settings.

NOTE: Be sure to disable Find My iPhone if you have it enabled, and logout of any iCloud services.