Photos on the iPod are stored in a particular way. During syncing, iTunes converts the photos to fit the iPod's small screen and stores the data in a special database format. This is to optimize how the iPod operates when showing the photos and to save storage space on the iPod. Like with other types of media, iTunes syncing in generally in one direction only, from iTunes library to iPod. You cannot use iTunes to retrieve photos on the iPod.
NOTE: The exception is IF you had a setting enabled when syncing the photos to the iPod. There is a setting on the iPod's Photos settings screen in iTunes (where you set up syncing for photos) to Include full-resolution photos. IF that setting was checked when you synced the photos to the iPod, you can find and retrieve the original image files in the Photos folder on the iPod storage (with disk use enabled), using your computer's file manager.
There may be third-party utilities that can retrieve individual photos from the iPod. However, the photos are typically lower-resolution versions of the original full resolution photos, because they were converted during syncing for the iPod's small screen.
