iCloud

Hello! I use iCloud to back up all of my devices but my Mac continues to tell me that my storage is almost up. I am assuming that means what is actually on my Mac and not in the cloud. I have a lot of pictures and i am finding that if I delete any of them off of my Mac they are gone and on my other devices as well. I need to free up space on my Mac and can't figure out what I need to do for that to happen without losing my pictures. Help!!