If you want to remove single photos, this is not possible. The iPod is used to "sync" photos, not be a photo organizer itself. You can sync selected photo albums in iPhoto or you can sync all your photos in iPhoto. You can adjust these options in iTunes's preferences (for versions 6 and earlier) or by connecting your iPod and adjusting the settings on the iPod settings screen (for iTunes 7).
If you wish to remove all photos, simply disable photo synchronization via the above methods.