Q: delete photos on iphone

That depends on how you imported the photos to your iPhone.  If you used iTunes to sync the photos from your PC to the iPhone, you can inly remove them by syncing again.

See: Sync photos from your computer to your iOS device using iTunes - Apple Support

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.