select photos to back up in iCloud storage?

Hi, I have an icloud storge for 50 GB. I don't want to buy more storage

But now I get a message that I am reaching this 50GB. most of it are photos.

Can I select in iCloud which photos I want to back up or is it all or nothing?

I have enough space on my iphone to keep all the photos, but I don't mind to have not all in back up.

I hope somebody can help me