To delete photos that have been synced from a computer that's no longer accessible:
Create a folder on the computer that you now sync with, and add a single photo to that folder.
In the Photos tab in iTunes, click the box next to "Sync Photos From."
In the pop-up menu next to "Sync Photos From," choose the folder you created in step 1.
Apply the change.
Uncheck "Sync Photos From."
Apply the change again.
Your synced photos will now be removed from the device.
Last Modified: Feb 18, 2015