Confirm your Settings > Photos & Camera > I presume iCloud Photo Library is turned on.
Below that you have a choice to: Optimize Your iPad Storage or Download and Keep Originals.
The Optimize storage selection manages dinamically the amount of storage used for your images depending on how much free storage is available on your device. It does not mean that no images are stored locally. It does mean that as your storage becomes more limited, Photos will start reducing the quality (size) of the images stored locally, until, eventually, only tiny thumbnails will be stored on the iPad.
This is all done automatically and you don't need to "supervise" this. Unless something goes wrong :-/
Images stored in iCloud Photo Library are always stored at full resolution. Only iDevices and Macs have the option to use Optimized files.