If they are not in the DCIM folder, or a subfolder within, then they've been synced via iTunes. Normally that means they are already on your computer.
If not, I usually use PhotoSync (no hyphen). This and the companion software for the computer will transfer photo between devices and computers. There are other methods, but I'm not familiar with them.
Hold in mind that photos synced via iTunes are usually of lesser quality than the originals that were on the computer. Retrieving photos from albums in the Photos app is a poor second choice from finding the originals.
Unless you're looking at the Photo Stream. That's a different kettle of fish. I know they can be synced via iCloud to a Windows computer (Vista or newer), but I don't know the details of how. I have a Mac. Otherwise you can save each one you care about to the camera roll in Photos on the iPad, then use the DCIM folder to copy them to the computer.