I am always filling up my hard drive with photos from their digital cameras to iPhoto library. Now I would like to backup all my photos to icloud, not photo stream. How can I do that with iCloud and free space there?
You can’t. That’s not a feature with iPhoto or iCloud. And Photostream is not for backing up photos, and will delete photos from iCloud after 30 days. You can try other cloud service to back up all of your pictures, such as dropbox, sugarsync or google drive. The simplest way is to move your iPhoto Library to an External Drive and turn on Time Machine.