Photos imported from your camera using one of the Camera connector devices will be stored in a \DCIM directory on the iPod, which you should be able to access in Disk Mode. In fact, if you're using Windows XP with the default settings, it should pick up the fact that there's a DCIM directory as soon as you connect the iPod and offer to transfer those photos off for you.
Photos synced in through iTunes are stored in a proprietary database format, and may also have full resolution copies stored, if that particular option was selected. If the full resolution photos were stored, you can find these in the \PHOTOS directory on the iPod, organized into subdirectories by year, month, and day.
If you haven't stored the full resolution photos, XPlay Photo Browser
will allow you to extract images from these databases, but only at the resolutions they were stored in for viewing (so the best you can usually do is around 720 x 480).
For more information, I'd recommend the following articles/threads:
The Complete Guide to iPod photo Pictures
Photo Storage on the iPod photo -- The Gory Details