Q: If Photo Stream was preinstalled in my computer, do I really have to pay for an upgrade in order to use it with iCloud?

Why do you expect your photos to be on iCloud? What is it you're looking for exactly? What version of iPhoto do you have?

I would like to save the photos on iCloud

iCloud is very different from MobileMe. It does not store photos permanently. It will allow you to stream your photos to your devices, but it does not replace your iDisk.

What the Photo Stream will do: If you take a photo on a device it will upload it to the stream and buffer it for a month in iCloud, so that you can import it from iCloud to your other devices. After a month it will vanish from iCloud. To use the basic Photo Stream you need Mac with OS X Lion v10.7.5 or later and iPhoto 9.2.2 or Aperture 3.2.3 or later, see: iCloud: Photo Stream FAQ

Shared Photo Streams, available with MacOS X 10.82 and later, will allow you to create separate Photo Streams without a time limit, but also only a limited amount of photos. For Shared Photo Streams you need: Mac with OS X Mountain Lion v10.8.2 or later and iPhoto 9.4 or Aperture 3.4 or later

iCloud: Using and troubleshooting Shared Photo Streams

-- Léonie

Read these Apple documents to learn how iCloud and Photo Stream works:

iCloud: Photo Stream FAQ

iCloud: Backup and restore overview

OT

1 - PhotoStream or iCloud do not store photos and you can not view photos on iCloud - so what you want is not a feature

2 - PhotoStream requires iPhoto '11 so to use it you do have to upgrade which is a purchase - and to use Shared PhotoStreams you will also need to upgrade your OS to Mt Lion

LN