PhotoStream doesn't store photos. It syncs between Apple devices. There's a limit as well:
Up to 1000 images will be visible there, and a photo can stay in PhotoStream up to 30 days, before it'll disappear.
If you intend to save photos to iCloud, use iCloud Photo Library (came with a software version of iOS 8), or use another kind of online storage for your pictures, e.g.
I had 400 or so photos that never disappeared, they were just there, and one day they were gone. What made me notice they were gone is because I was about to back some very important ones to CD. I had my iPad mini since they were introduced and only got rid of it last month when I upgraded to a regular iPad. The pics disappeared sometime around six months ago.
Better not rely on PhotoStream. It's not designed to store photos forever. As you see, it's possible that your images disappear unexpectedly. If you have to erase your device, or PhotoStream gets disabled, they'll be gone as well.