I've had the same issue and couldn't find a solution online.
I found a perfect solution though, what i did was:
Go to '''Photos''' > ''' Computer Photos''' > '''Select All'''> '''Duplicate'''
This ultimately add all the computer photos to your camera roll where you can delete them freely.
As for the computer pics you no longer need, you can use the solutions provided before; one of which is to remove the sync in '''iTunes''', or syncing with a new folder and removing the old pics.
Hope this helps ! xxx