Yes. The backup only backs up what is on your iPad at time it is backed up. Apple indicates that it maintains up to 3 backups, however as you continue to back up, any backups that may still contain the photo will eventually be overwritten.
To save your photos, always import them to your computer (http://support.apple.com/kb/ht1808). This will back up camera roll photos (and videos). If you want to save photo stream photos, save them to the camera roll before importing (by opening your my photo stream photo, tap Edit, tap a ll the photos, tap Save to Camera Roll).