iCloud storage and optimising space on iPhone

HELP!!!

I have to get my photos off my phone as it has zero memory left! My issue is I have been using the iCloud function to optimise my phone storage and all the photos are just thumbnails and not the real images. All the real images are on iCloud.

My understanding is I can’t delete these photos off my phone because they will also be delete from the cloud. I can’t download the photos from my phone to the computer because they aren’t actually on my phone anymore.

I’m so confused.

Can anyone shed some light on what I need to do as I can’t figure it out! I need to make sure I have the photos 100% saved in their original size before I go on a deleting spree.