Your iCloud backup contains camera roll photos, which could be recovered by restoring your phone to the entire backup. Photo stream photos, however, are only held in photo stream for 30 days. After this, they are removed from your photo stream even though they remain in your My Photo Stream album. If you reconnect to your photo stream, however, you will only receive the photos from the last 30 days as they others are no longer in your photo stream.
To save your photos, always regulary import them to your computer, as explained here: http://support.apple.com/kb/HT4083.