iPhone storage vs. iCloud

I want to delete the pictures on my phone to free up storage space. I have a large iCloud storage plan, but I am hesitant because when I go to delete a picture, I receive the message that it will be "deleted from my photo library on all devices." How can I ensure that I don't lose the pictures, since they cannot be replaced? Can I trust that once they are in the iCloud library, they will remain there unless I log into a computer and delete them from there?