You can do that (in a useable form) only if you had a particular setting enabled when you synced the photos to the iPod. To find this setting, select the iPod in the iTunes sidebar, under DEVICES. Over to the right, go to the Photos tab. On that screen, there is a checkbox that says Include full-resolution photos. IF that checkbox was checked when you synced the photos to the iPod, they are stored separately on the iPod as individual image files. You just need to Enable disk use (on the iPod's Summary tab), and use Finder to open the iPod's "disk." The image files are in a folder called Photos.
If that checkbox was not checked, the photos on iPod only exist in an optimized format to fit the small iPod screen and they are stored in a special "database" file (to save as much storage space as possible on iPod). Therefore, they are not in a format that is directly usable, and they are low resolution, so even if they were extracted (using a third-party utility), they would look highly pixelated when enlarged to computer screen size.