An easy check. Go into the settings on your iPad, and under your iCloud account, check the setting for "photos". If there is a setting for "my photo stream" and it is on, then you are using Photo Stream - My Photo Stream FAQ - Apple Support
Note from that FAQ that Photo Stream never used any of your personal iCloud storage anyway, so buying more personal storage does not increase space for photo stream. Photo stream is a system intended for synchronization of images across multiple devices and was never intended to be used as permanent storage or backup. It is a temporary parking place for images to allow for them to be shared across multiple devices.
With iOS 8.3 and OS X 10.10.3, there is a new service, iCloud Photo Library which actually IS online storage of your photo's - all your photo's. It does use space in your personal iCloud account. Since the default free storage is only 5GB and is used for your iCloud mail, backups, synchronized files and data, the default setting for iCloud Photo Library is to keep all photo's on the device as they are, in order not to fill up your free iCloud storage. If you purchase additional storage and want to remove photos from an iDevice or computer, then you need to change the settings on the device to clear out space once it has uploaded images to your iCloud account.
If, in your iPad's settings, general, about just look at the "version" line. If it is not showing 8.3, then you cannot possibly be using iCloud Photo Library and must be using Photo Stream. Until you upgrade to iOS 8.3 you would not even have the option to use iCloud Photo Library as it is a new service, introduced with iOS 8.3.
If you are running iOS 8.3, under your iCloud account settings, under "Photos" you will have three settings:
1. iCloud Photo Library - enable this to use your personal iCloud account storage for cloud storage of your photo library
2. My Photo Stream - enable this to synchronize photo's across all your devices that share the same iCloud account. This does NOT use any of your personal iCloud account's storage. Images will be automatically deleted from your stream after 30 days.
3. iCloud Photo Sharing - allows you to create albums which can be shared (viewed) with others.