Only the Camera Roll, Imported, and Saved Images folders can be copied off of the iPad via the USB method. Photos in other albums were synced via iTunes, and can only be changed with the sync settings in iTunes, or modifying the folders on the computer that you are syncing with.
You can email up to five pictures, no matter what album. If that is not enough there are several other apps that can copy pictures from the photo albums. My favorite is PhotoSync. Hold in mind that photos synced through iTunes may not be as high quality as the originals on the computer. iTunes optimizes the photos for viewing on the iPad before syncing.