Photo Stream is a sync tool not a backup. You should transfer your photos to your computer to be backed up.
Photo Stream only holds photos on the server for 30 days and only holds 1000 photos on your iOS device. The oldest are deleted after the limit is reached. If they are synced to your computer then there is no limit, however if you were to sign out of iCloud on the computer it would remove all the photos in Photo Stream.
Again, it is always best practice to transfer your photos from your device to a computer and include them in your backup routine