How can I delete photos imported to an iPhone from a PC?
Open iTunes and select your device by clicking the device button in the top-right.

If no device button is shown, make sure your device is connected (whether by USB or iTunes Wi-Fi Sync).
Select the Photos tab.

There should be a list of sources that you can sync from.
Uncheck the albums that you don't want syncing to your device.

Alternatively, uncheck the Sync Photos to remove all imported photos from your device.
Click Apply (or Sync) to sync the changes to your device.