I accidently imported photos from my pc onto my iphone and cannot delete them?

You need to un-sync the photos from the phone using iTunes.

Keep your photos safely stored and up to date on all your devices - Apple Support

Turn off iTunes syncing

If you don't want the synced photos on your iPhone, iPad, or iPod touch anymore, turn off syncing in iTunes and they'll be removed from your device. You can turn off syncing for some of your photo albums or all of them at once.

Stop syncing some albums

1. Open iTunes on your computer and connect your iPhone, iPad, or iPod touch.
2. Click on the device icon in iTunes.
3. Click Photos.
4. Choose "Selected albums" and deselect the albums or collections that you want to stop syncing.
5. Click Apply.

Stop syncing all photos

1. Open iTunes on your computer and connect your iPhone, iPad, or iPod touch.
2. Click on the device icon in iTunes.
3. Click Photos.
4. Deselect "Sync Photos" then click "Remove photos."
5. Click Apply.

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