Photo Stream is not a good choice for photo backups. It intended as a way for you to easily see all your recent photos on all devices, and save them to that device if desired. Photos are only saved for 30 days or 1000 photos. Older photos are bumped after one of those two limits are exceeded. There is not long term photo storage on the iPad at this time. Well, not entirely true. The new iPhotos app will let you create and share journals (pages of photos). Those are stored in iCloud.