IPhone5C

If you used iTunes to sync photos to the phone from a computer, the instructions below from Sync photos from your computer to your iOS device using iTunes - Apple Support describe how to remove them:

Delete synced photos

1. In iTunes, locate and click on your device.
2. Click Photos.
3. Choose "Selected albums" and deselect the albums or collections you want to delete.
   To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
4. Click Apply.

Learn how to delete photos and video synced from iTunes when you can't access the originals.