You are better off backing up to the following cloud:
You might find this article useful: http://support.apple.com/kb/HT4859
I do, however, agree with Diavonex. You should look into additional cloud storage for those photos rather than just relying on iCloud.
iCloud and Photo Stream are not a good back up solution, it is made more for keeping devices in sync rather than backing them up. There are other back up solutions on the cloud but the best option is to get the photos backed up to a computer and it's back ups as well. Once on the computer they can be organized and selectively synced back to the iPad if you wish. Even though Photo Stream should not be considered a back up Shared Photo Streams may be used as a method of getting some of you favorite photos back on an iOS device.