How to safely remove downloaded iCloud photos on my iPhone yet leave them in iCloud

I know that my phone uploads my photos to iCloud to save space on the phone but I have a question concerning photos that I have re-downloaded from my iCloud photos backup. I pulled some photos back from iCloud onto my phone and I am done with the larger-sized images on my phone. How can I safely remove them again from the phone and avoid deleting them from my iCloud photos backup? I just want to free up the space on my phone again, yet make sure the images stay in my backup on iCloud.