If the Photos are in the Photo Rolls, once you since the iTunes to the computer, it should pop up a folder (in Windows) and you can delete the files like normal files. Not sure about the steps in Mac.
If the Photos are in Photo Library, You would need to is to plug the iPod Touch to your computer and in iTunes, go to the devices tab and under 'Photos', you either uncheck the Sync option or Sync it to a folder which does not contain photos and do a Sync or Refresh. This will prompt you if you want to delete the photos on the iPod touch.