If you copied photos to your iPod using the full resolution option, enable disk use, open your iPod in "my computer" in Windows or Finder on a Mac, open the photos folder then open the "full resolution" folder and drag the photos out of that folder to your desktop. For enabling disk use look here.
Disk Use.
If you only copied the photos as thumbnails, you can't do this so you'll need a program such as this one.
iPod Photo Copy. (Windows only)
iPod Photo Liberator. (both Mac and Windows versions available)
Photos as thumbnails won't be of the original quality when copied back to your computer.