using iCloud to free up space on my mac

Deleting from a device where iCloud Photos is turned on will delete them from that device, from iCloud and from all other devices synced with iCloud Photos.

You can free up space on the Mac by turning on the following setting in the Mac Photos app: Open the Photos app, click the menu item named Photos, click Settings< click iCloud at the top of the Settings window, turn on Optimize Mac Storage (read the fine print there -- see example below; note that Optimize Storage is off in that example)

Do you regularly backup your pictures? Do you want them safe? Backing up Photos is so easy-- you can use Time Machine, and you can also just copy the Photos Library to an external hard drive. I do both.

"Optimize" saves space, because the Original pictures are not stored on your Mac. Instead, Photos depends on getting them from iCloud when they're needed. Using "Optimize" may save as much as 90% of the picture storage. But the downside is that this means that a your Mac won't be able to back up your pictures, because they're not all there.

Probably a better option for you is to add an external drive, which works really well on an iMac. I'd keep the Photos Library on the internal drive, and move documents and such files to the external drive. Then you don't need "Optimize," and backing up your pictures is easy. You'll feel way better.