There is a setting that needed to be enabled when you synced the photos to the iPod, if you want to recover the original photo files later from the iPod. That setting is on the iPod's Photos tab in iTunes, where you set up syncing for photos. It is a checkbox that says Include full-resolution photos.
If that setting was enabled when you synced the photos to the iPod, iTunes puts a copy of the original photo files in the Photos folder on the iPod. The iPod does not use those image files for displaying on the iPod's screen; they are there as a backup copy.
If that setting was not enabled, iTunes converts the photos during syncing to optimize them (make them lower resolution) for display on the small iPod screen, and stores the photo data in a single database file. This is done to save storage space on the iPod, and make it easier for the iPod to display the photos. There may be a third-party utility that can extract individual image files from that database file, but the result will probably be very low-res compared to the original photos.