I don't have a lot of photos on my camera roll on the iPad, however there are a lot of photos stored on the Photo Stream section that I'm thinking is going towards the backup. I'm not entirely sure how to delete those photos from the Photo Stream. I like the Photo Stream option however I wish thee were an option to remove certain photos from it. If there is a way to delete specific photos from the Photo Stream I'm unaware of how, as I'm still "learning the ropes" of this whole iCloud system. As far as once the 5GB storage limit has been reached there will not be any backups that will begin again and override previous backups. This is the email Apple sent me:
As a reminder, iCloud comes with 5 GB of free storage. You are currently using 4.2 GB, which means your iCloud storage is almost full. When your storage is full, device backups to iCloud will stop and apps will no longer be able to save documents to iCloud.
Dear ****
To ensure your iCloud services continue without interruption, you can buy more storage or free up space by following the steps below:
Step 1: On your iOS device, go to Settings and tap iCloud.
Step 2: Tap Storage & Backup.
Step 3: Choose Buy More Storage to upgrade your storage plan, or Manage Storage to free up space in your account.