Hi, welcome to iPF!
You can't decide where to save pictures on your iPad. All of them will be stored in your Camera Roll, which "sits" on your device.
If Photo Stream is enabled, they automatically are visible there too, but PhotoStream is "in the cloud", which means that your pictures are saved in iCloud, and can be accessed on all your Apple devices. Also, it "only" shows the last 1000 pictures from the last 30 days from all your Apple devices where you've enabled this feature.