Unfortunately, there isn't. iCloud only keeps photo stream photos in iCloud for 30 days, allowing enough time for them to stream to your other devices and be saved, if desired. After that, they are removed. They aren't included in the backup because they are assumed to already be in the camera roll (which is backed up), since all photo stream photos originate from the camera roll. The problem comes when someone deletes them from the camera roll, assuming that they are backed up in photo stream, when they aren't.
In the future, you should always import them to your computer while they are still in the camera roll, as explained here. (Alternatively, if you have a Mac running OS X Lion or higher with iPhoto 9.4 or higher, you can set iPhoto to automatically import your photo stream photos to your computer from photo stream.)