apple would appreciate it if you did it this way.
go to settings iCloud
enable iCloud photo sharing
enable my photo stream
enable iCloud photo library
doing it this way is going to cost you a monthly fee. you only get 5 gb of space for free.
apple wants 99 cents a month for 20 gb, $3.99 a month for 200 gb , etc etc
as long as you either pay the monthly fee or live with a puny 5 gb of cloud storage, your pictures will be safe
on iCloud for as long as you keep paying. once they are on iCloud photo library , you should be able to delete them
when deleting pictures i think you limit yourself to deleting pictures in the camera roll.
if you delete them from other places like photo stream or the iCloud library they will be deleted across devices
there are other cloud places to store your photos. so explore them
watch out though. some of these other places that store your pictures can resell them.
whatever you do. start syncing your iPhone to photos / iPhones on the mac , or windows, iTunes etc.
you want to make sure your photos are backed up on your computer before you start messing around
and deleting them from your phone. you don't want your phone to be the only place that has that picture
i thought how photos in the cloud works is you take a picture and the high quality one gets put up on iClou and then you get a smaller picture on the phone, or the picture gets deleted from the phone automatically but i haven't signed up yet for this
photo stream is totally free. and whatever pictures you take get put there. but the drawback is apple keeps 1000 pictures at a time. if you wait too long, apple will eventually delete the picture from photo stream. this is why its so important to back up photos to your mac or pc , no matter what solution you choose.